It seems pretty obvious; never assume that somebody will play a certain way until you have solid proof of it. If everybody plays a lot of hands yet nobody notices I'm playing super tight, I'm fine with only playing top hands as long as I still get action.

The only time you may want to assume is when you first enter a table with people you haven't seen. In that case, maybe for a few hands I'll act like everybody's loose passive, but I quickly find out more and adjust appropriately.