You are correct that pushing a 55% or any edge is +EV but I disagree that that means it's ALWAYS the best decision to push it. Chip Reese explained this concept way back in the original Super System book by Doyle Brunson. He said he kept pushing marginal edges against a donkey one night in Vegas and ended up losing his money for the night because the edge didn't work his way while other players ended up busting the donkey. He knew he could have outplayed him in much better spots but got over anxious trying to push small edges that went against him. He learned from that situation to be selective about how you attack a fish and not just jump on the first marginal opportunity that comes along.

Say, for example, you're against a loose, maniac who will stack off with just about anything, and he keeps pushing all-in pre-flop and you find yourself with pocket deuces. Your only options are to call your stack or fold. You know he has any random two cards and you're the last to act and everyone has folded. If you call, you're probably a little more than 50% which IS a favorite and WOULD be pushing your edge. But, would you call?

In a tournament with escalating blinds, you'll probably have to call with Jack high in many situations. But, in a cash game, you decide when you want to take advantage of what situation. You can call off your pocket deuces now or wait till you get pocket Queens and now you're an 80% or better favorite.

If you constantly find yourself pushing marginal edges over big ones, you'll be playing a lot of break-even poker.