Many times during a beta, developers will ask their testers to concentrate on a certain feature or give feedback on an area they are working on. Other times, they just want general feedback - and of course, all the time they will want bug reports. Anything you run into as a tester that doesn't act right should be reported right away. Giving the developers the path you took to reach the bug (the "steps to reproduce") helps them locate and smash the bug quickly.

I've tested some more recently and didn't get to file any bug reports - only reported two minor things that I thought might need looked at. This is a good thing.