first off, i don't vary my raise bets because every once in a while you find an observant opponent who will notice that.
As for stealing, it depends on the table and the blinds you are stealing from. if they are tight, it doesn't matter what cards you are doing it with and you may be able to do it almost every round. If they are tight but good, you need to not do it TOO often as they will then be looking to trap you. Sometimes its better to do it with non Ace hands and such just in case you get a call because they will often call with hands that dominate you but if you do it with something like 98, you can hit a good flop and they don't see it coming.