You just have to be really careful. I have found this to be a pain myself. Last year i had to cut a 2 hour 5 camera gig. I had my 5 separate video tracks and audio tracks and also my master audio track.
The key for me was checking over and over again. Its the one thing in Vegas i dont like very much. It would be a great improvement if there were some better audio syncing tools.
It has come on in ver7 from 6 where its easy to see when video goes out of syn with its own audio as it changes colour. its an idea to keep the audio tracks there muted to make sure the video is in the right place. If you see red - its out of sync.