Everyone has a different approach, and you'll probably hear a few of them from other folks on this site. The one common thing you'll hear, is that there is no rule that applies in every situation. What gear are you using? What's your sound source? Where is the mic positioned? How will the sound be used once you get it back into the studio? What type of sound is it (transient, drone, etc.)? The answers to all of these questions (plus a few others) will dictate how you set your levels.
Personally, I frequently record sounds at the level I expect to use them in my projects and mixes. I use the tone generator in my recorder (1 kHZ at -20dBFS) to give myself a reference over my headphones prior to adjusting gain levels. It's a basic method for calibrating your gear out in the field...and you should always work under the tightest calibration you can (in the field or studio). There's no point in recording foley sounds or ambiences with peaks of -3 or -6 dBFS. That's not how I'm going to use them in my mixes. For sources that are really loud, I may want to let some distortion into the recording. It can give the sound a unique character (provided it's not overbearing), or the distortion could be of such a short duration that it is virtually undetectable (this can happen with transient sounds, giving you a better recorded signal for the tail out). I don't do it all the time. I make that judgement in the moment.
I know you're looking for a few simple guidelines, but the kind you're looking for don't really exist. There are no magic numbers that will work in a majority of situations. You have to think when you're recording. So, the rule is to ask yourself questions:
- How am I (or someone else) going to use this sound?
- Does it have the right sonic character for that use?
- Does it sound good?
- Is the signal as clean as I can get it in this situation?
Use your ears, answer those questions, and you'll be making great recordings in no time.