I usually get on the tops of the bars for speeds over 50 mph...

No stead and fast rule for when to get in the drops. If I'm going into the wind, hammering, etc. then I'll get in the drops. I occasionally climb in the drops when I want to really get my ass, lower back and hamstrings into an out of the saddle climb.

I pretty much use the tops of the bars or the hoods most of the time, but the drops are there for fun stuff. I've known guys that ride almost solely in their drops, though, so it really is a matter of preference.