To go with the guitarist analogy, Eddie Van Halen - one of the best, inventive, original (and granted, very talented) to ever play - has said many times that the only rule that his music teacher in college had was "if it sounds good, it is good".
I'd say the same applies to painting. If it looks good, it is good - technically "correct" or not. It is good to have some basis of tried and true methodology - it can really help when you can tell that something
doesn't work but you just can't quite put your finger on it. I also feel that others opinions help greatly - especially "non-artists" - even when I am happy with it or don't see anything "wrong" in a painting.
If you can ask them to go ahead and "crush me" and let them really be critical without getting offended or lose self-confidence, one can get a lot of diverse opinions on what "general" things people like or don't like to see in a painting - many times contrary to each other. Which should also go to show that it depends on the person as to what rules actually work or not. Sorta like music - some like the "rules" of rock, but not rap. Maybe country, but not classical. Maybe a big mix or range, and sometimes they mix well, and other times they don't.
It just depends.