Should I switch off phantom power before plugging in my mics that require phantom? I hear a click when I don't switch the machine/phantom power off. I tend to do it, but if I don't need to that would save me some time :-)
I would beware some ribbon mics - they can be a bit finicky with power - but on the whole, it's not a problem. I would make sure the channel is muted so you don't have potential speaker/ear damage from the pop. Beyond that, you're fine.
I've never seen a warning with a mic about phantom power. If it were a problem, you'd think we'd be warned ;) I do it all the time with a variety of mics and I've never had an issue.
I don't think it's much of an issue.
I've been taught much like Nathan to plug in mics and then powering on the preamp.
This is EXPECIALLY important when handling older condenser mics which have their own outside power supply, like a U-47 or U-67. Always plug in the mics with the power supply DISCONNECTED from the wall because you never know about those 40 year-old or older power supplies... maybe that switch doesn't work... A wise investment, because I've seen someone tampering with an old C-12 mic (disconnecting it with the power still hot) and getting shocked by it and thrown across the room and having a black thumb because of it.
But, those are the older mics.
Newer ribbons, if the XLR is incorrectly wired, can be fried by the phantom power - so you have to watch out. I have a little tester I use before I plug in any of my ribbons that tells me if the cable I am using is wired correctly. It's pretty common - Royer even has some warranty and will replace a mic after your first barbecue.
Newer condenser mics I am pretty certain wont be a problem, but I like to flip it off while switching mics because I like to keep my mics clean and in perfect condition.
I've always been taught to never hot-plug a mic, and to shut off phantom power when connecting or disconnecting condenser mics. Not sure if this is old-school paranoia or solid advice from my mentors, but either way, it's a habit I've developed. There's a somewhat he-said-she-said thread discussing this at Home Recording, and a more reasoned technical discussion over at Gearslutz. It does indeed seem to boil down to the design of the preamp and the mic, to some degree. Some Tascam users report frying mics on their US-122's, but I'd be shocked if anyone has done so between, say, a Sound Devices pre and a Senny or Neumann. (would love to hear otherwise, though)
The one thing that everyone seems to agree on is the issue of what happens when you plug a dynamic mic into phantom power! :-)
Much like @NoiseJockey and @Utopia, I've always been taught to switch off the phantom power. I also have many friends that do Audiotronics who say it's perfectly fine to hotswap modern mics, as they have better and more reliable resistors and capacitors in them that will prevent them from getting fried. Vintage mics are less reliable, have (Technologically/Safety) inferior designs and you should take due care with them and the only other thing you should be careful of is defective equipment or to be a bit extra careful with Ribbon Mics as they're a more sensitive and finicky component.
Just to echo what everyone else has said here, it's probably not a problem for modern mics (except maybe ribbons). If anything funky does happen, the ground pin is always the first to actually connect between your cable and the mic/pre. So, there's at least a path established to ground to help deal with low-level discharges. That is, of course, unless the cable's damaged or you're using a cable that has a "ground lift" at one end (more likely to come across that in live sound situations).
All that being said, I still do the cold swapping I was taught years ago. If nothing else, it protects your ears and your monitors.
Although I've never experienced a problem, I always try to remember to apply power after connecting the mic, and remove power before disconnecting. This text is from the product manual for the Rode NT1-A condenser mic:
"Connect all cables before applying power to the microphone and never remove the microphone cable while the power is connected."
i've never really had any issues with it, although it seems rather crass to hotswap a powered mic especially tubes and ribbons.... whatever you do you should ALWAYS mute the channel who's mic your switching, then you definitely wont hear a pop, you also definitely wont damage you amp, monitors or the eardrums of any unsuspecting musician who might have his IEMs in.....
Although I'm confident that "hot-plugging" a modern microphone shouldn't cause any damage, I do like to use logic:
In chemistry, it's against safety procedures to dilute acid by pouring water directly into the acid. The correct procedure is to instead, pour the acid into the water. It sounds silly, but the incorrect method could have serious repercussions depending on the acid.
Translation: Plug the mic in, then apply power.
There are several problems in connection with plugging a microphone into active power. Ignoring the mixer side, there is the obvious issue that connections may be established in arbitrary order. Good microphone circuits will likely not mind, but dynamic microphones (including ribbons) contain very little circuitry. Sure, you usually are better off not applying any phantom power at all, but not every mixer has that option.
The second less obvious issue is that when fiddling around with a microphone plug, wrong connections might occur. Will your microphone tolerate -48V instead of +48V on one of its lines? Or 0V on one line and -48V on the other?
Some XLR connections are reasonably safe against any mixup (and some microphone plugs even have the mass pin longer than the other pins to give some guarantees for connection order, but obviously that doesn't help if this is not the last connection in the line you make).
So why take more risks than necessary? To save a few seconds of time? How much are you going to save in that manner?