I think I rode 90% of my mellow 1.5hr ride today on my hoods (flat, flat, flaaaaaat), but that said...

If there's a "bump" in the road (i.e., something steep enough to make you think about changing gearing/position/etc but really short), I'll stay on my hoods and stay on the same gear and just stand up. I've noticed from group rides (when I'm surrounded by cyclists who know soooo much more than me) that this is a common tactic...it allows me to keep up speed because you don't lose any time on gear changes. Plus, getting out of the saddle is just fun!

If it's longer or steeper or just a plain old HILL, I definitely go for the tops of the bars and stay seated. It opens up your breathing and puts your hips in the right place for optimum climbing power. I've talked to some really knowledgeable people about this. I definitely notice that when I go to the tops of my bars I can climb more efficiently. Then mix in a some standing up for variety...

Cheers!