SmartGo has this feature, and I find it extremely helpful. Once a move is marked as good, questionable, or whatever, then whenever you see its letter on the board (depending on your settings, probably when you're at the node just before it), you see the appropriate symbol next to it. So instead of seeing "A", "B", and "C" for three variations after the current move, you see "A", "B??" and "C!" or what have you.
Drago is the best freeware go sgf editor out there. Thanks!