Not that you shouldn't try and improve, but I've always admired those singers who know their limits and sing accordingly. Like Mark Knopfler (Dire Straits) for instance...alot of the time it's just barely above regular talking. And I know most players who sing are pretty self concious. I remember reading both Hendrix and Stevie Ray were wary of singing at all, and were very critical of themselves. Of course it didn't hurt any of these players that they're geniuses on the axe!