The ash doesn't really take a light very well, so shouldn't contaminate the taste much for most of the better half of the bowl, at least in my experience. Sometimes when the ash gets thicker I get a little ashy flavor, I think this is due to longer application of the flame and probably drawing harder when attempting to re-light through the ash. Usually only happens once a bowl, if that. I find that I have a tendency to "over-light" on re-lights through a thick layer which sometimes makes the baccy taste bad also.
Sometimes on a deeper bowl, the ash can get thick enough to make lighting the tobacco underneath difficult and I'll dump to get a better light even though the flavor isn't effected that much.
Also depends on the baccy and whether or not I have dgt'd at all. Some tobaccos (flakes and similar stuff) leave a hard crusty ash if allowed to cool too much. I'll stir this stuff sometimes to make lighting easier and find I get a better flavor than just lighting through the crust. Other tobaccos (shag, ribbon, cube) don't really leave that hard crust and the ash is looser and lighter and so don't require as much finagling to get it how I want it.
Just keep chasing the flavor and do whatever you gotta do! No rules there as far as I know. As long as you are at least a little carefull with the flame and don't go scraping around the inside of the bowl with a crocodile dundee knife or something, you should be good!