I prefer not to. People say "It's not that big of a deal!", but reality is (when you've studied diseases long enough to understand), you already have enough germs on your body and the last thing you want to be doing is sharing germs. I try to stay healthy. I don't really share drinks and still get sick quite often. Don't do it. :)
I don't mind sharing my drinks, but there are people out there that won't even touch the rim of a drink with their mouths. That drives me nuts, but in the long run, it's their health. They and you should do what feels comfortable.