it's not what hands you are holding that should indicate whether it is safe to go all in on the flop. It's what your opponents have done (which obviously depends on your position), and what the actual flop is like. Sorry, but this is far to general of a question to have a specific answer. The problem is that if you flop quad 5's against one other limper, and the board is K55, you probably don't want to go all in because they will fold and you'll be losing a chance to get some extra money out of them. Seems strange to try to define exactly what hands are willing to go all in. In rare situations I might do it with high card Ace on the flop. You never know. In other situations I could hit a straight with 1 gappers on a rainbow flop and have one or two aggressive bets into me, and still not want to go all in.