In my experience when I've mixed rap vocals they tend to have a pitch. Usually the rapper will sit on the key note of the song, which makes it easier to mix as you can expect what frequencies will peak. The only time you might want to think about mixing it differently is if the rapper starts to get quite breath-y, in which case you'll want to watch the high frequencies and duck out quite a few notches. This is because the voice can very easily become grainy when compressed or saturated.