some genres use very predicatable formulas, some don't, but i think it's better to know your individual records inside out. there is always a range within each genre anyway. instead of grabbing two records at random, and then discovering in the mix that the basslines clobber each other and the changes don't line up, you can do things like mix a simple beat with a complex one. you should really have an idea of what it's going to sound like before you even put the record on the platter. place your sounds intentionally instead of mashing them together and hoping for the best.

i've seen lots of drum & bass dj's do that. two rollers together would be a nightmare, but two steppers together is boring, so they will mix a roller and a stepper, and magic happens.