I would say the closer the likelihood that you'll be in a suit and tie, the closer the shave should be. The closer you are to a hard hat, the longer the beard can be.

Some things are a toss up. If you have a professional certification, I would take my cue from the owners or co-workers where you applied. For instance, I don't see a lot of RN's with long beards, but I do see a lot of HVAC guys with them.