I like Ambrosyne's Vent chart, even if it is a little harsh. It's true, when you're not part of the raid leadership (and especially when you're a talker like me), it's sometimes a little hard to tell whether it's ok to try and crack a joke in the middle of the raid on Vent or not. Usually: no. I've learned the same thing by my own experience -- no matter how easy the fights are or how your guild is doing, the raidleader probably won't appreciate you trying to play some "Safety Dance" over Vent during Heigan's dance-off.
However, in my experience, there usually is a place for joking during raids, and it's the guild chat channel. Actually, that's where I do most of my communicating -- I leave the Vent chat open for raidleaders to address the whole raid with the actually important stuff, and the rest of us hang around in the peanut gallery of raid chat, grats'ing each other on good loot and slipping in some commentary for the raid. You've got to have fun, right?
Raidleaders, are there any good times to open up Vent and let everybody talk? And everybody else: have there been problems in your raids thanks to an unwanted Vent outburst, or does your guild pretty much understand that silence is golden?