from what i gather, don't pop him until you're ready to actually research the tech. if you pop him now for part of a tech but don't finish it, then research something else in the meantime, your population might grow and you'd have gotten more beakers for popping him later. unless of course you whipped away 4 bodies in every city i suppose...
i see my ALC hero popping them for something he's going to research in the future, but not changing to finish that research yet, all the time. he stays on his current research, then goes back to what he part-bulbed. from what i understand that's not the way to do it is it?
sidenote: the hammers that a GE contributes to building a wonder are based on the size of the city you're building it in, by the way. the early wonders they'll finish off in any city pretty much. but the late game ones, sometimes a city isn't big enough for one to do the whole job (pentagon, UN, 3GD i've seen it happen). or say, versailles in a size 3 city you just captured, where it would be oh so perfect, but doesn't really have the capacity to build or whip it yet arrrrrrgh. that's not based on your empire, that's purely the city itself, in a bigger city you'll get more hammers from him.