With Rap Vocals - or any speech that will be up front in your face - you want to edit out things that are potentially offensive or annoying like ...
- Lip Smacks
- Excessive Sibilance
- Excessive breath noises
- Mic bumps or any other unwanted noises like digital clicks
Because rap vocals are typically very highly compressed in a mix, all of those kinda of things will become even more obvious and more audible once you start to compress the vocal. If the quality of the recording is sub par, it can be a very tedious thing to do, so pay attention to recording methods and techniques and mic placement, be patient during recording, to keep that kind of tedious post editing to a minimum.