The reasons behind setting it to above normal are pretty obvious; To catch things that haven’t been discovered yet. There is an obvious setback of a potential False Positive (that current Avast users seemingly haven’t had problems with too much, judging by the responses so far).

If you’re willing to risk the fact that you might have a false-positive occasionally while making sure that your computer might not get the latest computer virus, then go set it to HIGH. If not, keep it at NORMAL and you probably won’t have to worry about FP’s too much. then again, you might get a new virus.

Up to you I guess.