I need to start by pointing out that what we are talking about is absolutely no different for rap/hip hop thanfor any other genre...
What you are talking about is really still "mixing"...
Peoples misunderstanding of the term "mastering" is really starting to work against you...
Just because "mastering" includes using compression and eq does not mean you are "mastering" every time you apply these processes (i.e., "should I master my vocals?", "should I master my beat before I put the vocals on it?", etc)
just because you put a compressor over you master bus when you mix, doesn't mean you are "mastering", too...
If you are making a beat and you are ready to put vocals on it, think of the vocals simply like another instrument in you mix-- because that is exactly what it is.
If you got a beat from someone else and it sounds dull and lifeless and you can't hear certain instruments-- that means the beat was not "mixed" very well (or at least not to your personal tastes)... If a beat is mixed well, you would not have problems like this...
When you have this beat in your sequence and you are applying eq or compression to it (or whatever) to make it work the way you want it to, you are really "mixing" it into your song... Just like how if you are using a sample from a record/CD and you eq it or compress it so it works better, you are "mixing" your song-- not "mastering" the sample.
Just because you are applying eq or compression to multiple tracks, it doesn't mean you are "mastering"... Like if you grouped all your drums together or all your vocals or all your synths and applied eq/compression to the group, you are not "mastering" those elements.
"mastering" is the final process applied to a completed production.
If you are in the business of "making beats", then the "beat" is your[\I] final stage so it is understandable that the question of "mastering" would come up in your mind...
Well, you want your beat to sound as good as possible, but this really comes from making a good "mix"...
I understand that most of you are not good mixers yet, so you apply a "mastering plugin" in an attempt to make it sound better, but your mix should really be doing that.
It is ok to apply eq/compression to your beat, but I would advise against doing too much of that otherwise you will be tyingthe hands of the person you are giving your beat to...
Anyway, back to my earlier point...
When you eq/compress a beat that you are using in your track to simg/rap over, what you are doing is mixing that beat into your track... Not mastering the beat.