this is a good question. have you ever watched the pros or seen any videos? if not, this is a good way to see their hand positions. in case you dont know about it, you can watch free cx videos here:
i will say that in general, you will be on the hoods 70-75% of the time, in your drops 10-15% of the time, and on the tops 10-15% of the time. and yes, the hoods are for the technical stuff, hammering in and out of the saddle, and dismounting. i find that the cross levers that some bikes have come in handy for dismounting and for negotiating steep/technical/muddy stuff -- especially if you are coming from an mtb background. when i first rode with drops i felt very unstable and even unsafe because i was so used to my mtb flat bars and brake levers being right there. so, for this reason i felt much more in control having cross levers, even if they did act as a bit of a crutch for me until i got used to braking with traditional road levers.
as for riding in the drops in a cx race, some will use them alot more than others (some on this board will say u use your drops *most* of the time), but i really only use them if i get a chance to use them -- which means on flat straight-aways and/or paved sections, and sometimes in the starts. personally i've never been in a sprint in a cx race, but that would certainly be a situation where you'd be in your drops (i'm looking forward to the opportunity).
hope that helps...have fun!