For mortal racers, I say stick to relatively hard wheels, and don't change em. That is what I think because it is one less thing to worry about. Durability comes first in my opinion, so stay away from super hard ABS wheels, unless you drive very clean and have a nack for avoiding those fast full speed clips.
Soft wheels are meant for really low traction tracks. Something to look into for slippery or concrete tracks. But I personally try to stick with a type of wheel and stay with it because I feel that you can get more advantages elsewhere. If I was a pro driver, I would probably beg to differ as they look for advantages anywhere.