Being relatively new to production sound mixing and post production sound, I've been wandering my way through the indie world not doing too much studio work. How important will it be for me to be a union member to begin working on large-scale projects? What are the unions or professional societies that I need to focus on? Are they a source of work, or more of a source of credentials?
Production Sound Mixers - very important - if you want to be playing with the big boys (Devlin, Novik, Ulano, etc...) on union sets. Union sets generally won't hire you unless you're union. The only catch is, once you're union, you're only allowed to work on one or two non-union sets a year. So you'd better make sure you're good and are plugged in before you go union, or you'll be out of work!
As for Sound Designers, I don't think its really that important. I don't know of many that are union. There are organizations, such as CAS, that a lot of Sound Designers (and Production Mixers) join, but those aren't usually unions.
Personally, I think unions have outstayed their welcome. They were very useful pre-war, but I think they do more damage than good anymore, IMHO.
The union here in Los Angeles (MPEG Local 700) doesn't recognize the title "sound designer" as an official classification. The closest you would get would be either a sound editor or a re-recording mixer. (Guess the "sound designer" fits somewhere between those two.) So with that in mind, you'd need to figure which of those best fits your skill set.
AMENDED: Without a union membership you're not able to be on union payroll. That being said, I think independent sound designers can work on union shows but cannot accumulate hours towards pension, health benefits, etc .
As a "PS" to Colin's opinion, I'll say that although the union certainly isn't perfect, it is the way the industry works. On the plus side, the health benefits are great - and that can be worth more than money.
if you wish to work on Union films or TV (which are pretty much all the big films and TV shows), you must be in the union- there are some workarounds, like having a very specific skill that is not available from the pool of union talent pool, but that will simply allow you a 30 day period of employment which will satisfy the union for membership.
The other clear way into the Union directly is to organize a film or TV show. That will allow all the crew to be admitted directly.