For mine it depends on the snake. Alex, my pied, is very well mannered and he'll tolerate just about anything but a direct touch to his head. He prefers a nice slow stroke along his body. Most of my snakes aren't as touchable as him, but even my most skittish one loosens up when I stroke her back. It calms her down when she's scared.

Whether or not they like it really depends on how you touch them. Most of my snakes don't like abrupt movement or being touched on the head or tail, but give their backs or sides a nice slow stroke and they'll be fine. Go with the scales, try not to venture too close to the underbelly, and basically just read the body language of the snake. You can tell when they're unhappy.