before you determine whether or not you need a compressor, first determine whether or not you have developed your playing enough to be able to control your dynamics with just your hands. you have to be careful, because you don't want to get to a place where you are completely reliant on a compressor when you play. a lot of players forget how much dynamic control we have with our hands. this happens because we are so used to playing through an amplified source, and subsequently can get conditioned to regulating our playing volume solely with our gear. i'm kind of in the same mindset as victor w. in that i agree you have at least the main source of your dynamics come from the hands, themselves. keep in mind compressors are 'dynamic processors' and in effect shrink the threshold of dynamics you are working with by applying a buffered ratio that limits the resultant perceived 'volume'. for this reason they are great for smoothing out transients if you are trying to seat the bass a little better in a track or in a mix. however, you want to be careful not to use them as a crutch.
there's a great exercise that i like to get my students to do when they are working on their dynamics. i first have them play a familar groove using fingerstyle, and then i have them play the exact same groove using slap techniques, instead. i then challenge them to match the volume of both versions of the groove, having them alternate the technique back and forth for each pass without stopping. it's amazing how quickly players will compensate by either lightening up on the slap 'effort', or digging in a little more with their fingerstyle. the goal of course is to try to get them very close. of course, the sounds of both versions of the groove will still be different due to technique, but you can successfully match the dynamics if you pay attention to how your hands are working together.
like any other effect or processing, too much compression can be a bad thing. a compressor can be used as a great tool, but if too much is used the bass tone becomes 'squashed' and will even lose its roundness. it's also a great idea to experiment with playing through many different types of compressors. some of them actually do color your tone (i've got an old HHB tube compressor that does a really nice job of adding subtle body and growl to my bass), and personally that is more of what i use a compressor for on sessions in which i am targeting a particular vibe with my tone.
anyways, a bunch of rambling but i hope that helps somewhat. comments that have been posted before mine offer great feedback, as well. happy practicing!
adam
