I just noticed that the very first reply to the OP addressed the first of my two (somewhat related) "Don'ts" (don't get straight below the rack). Apologies for the redundancy.
Let me elaborate briefly on my second point. I find that because my competence in controlling the distance of draw shots is less than great, I just don't have the full skill set to always safely work in the relatively small space the exists below the rack, especially when most of the balls remain on the table. Sometimes, of course, the only sensible, or possible, place to work is from underneath. But if the option exists, working from above often does not require quite the same level of precision, and allows you to more easily keep the cue ball free. Little mistakes, when playing from below the rack, can cause you to get stuck, or to end up going down table as a result of having to play a thin cut shot into a corner pocket. As for playing from above, you need to be prepared to waste a break ball in order to separate the cluster. Funny how the open shots from above, at least early in the inning, seem to so often be break ball candidates, ones that you'd rather not take. (The other funny -- or not so funny -- result that I experience too often is ending up right back in the same spot from which I just played a shot, only this time, without the option of playing the ball I just pocketed. Seems like one of the punishments doled out by the pool gods to those of us who are slower learners).