Have fun.

Maybe you're breaking a "rule" from a book, magazine article or web page. If you're having fun, don't worry about it.

Hack and slash is fine if everyone involved likes it. Interacting with store keepers is fine. Angsting away in the corner is fine. Backstabbing party members is fine. Using out of game knowledge is fine. Strongly plot driven games are fine. Open ended games with minimal plots are fine. Heavily planned games are fine. Games run entirely improvised on the spur of the moment by the GM are fine. If you're having fun... it's fine. Conversely, if a particular element is hurting the groups fun, take it out. Nothing is sacred.

For every bit of advice, guideline, or rule for better GMing, there is at least one game in your city, probably with people you know, that would be ruined by it.