The torque converter being a fluid coupling can handle slight rotation in the opposite direction when you change from Reverse to Drive and vice-versa. That said, for the stall setting of the torque converter, it doesn't take too much for it to be at the point that it can be considered a little too much and can put a little extra stress on things. You usually notice this when it gives a bit of a jerk. If when you change from R->D while still rolling back a little and it feels like it is engaging smoothly, this is what I would consider to be within safe boundaries. But that said, in all situations, my recommendation is that you should make sure you have come to a complete stop before changing from Reverse to Drive or vice-versa. But if you have a little bit of roll and it doesn't cause the car to jerk when engaging, then I wouldn't be getting worried about it. To answer you other question, when reversing, I usually change from R->D the moment before the car comes to a complete stop. I release the brakes, and from the time taken for the transmission to disengage from Reverse and engage Drive, the car would have rolled back no more than 30cm or so. If you get what I mean. This isn't personally something I would recommend doing, but it's a force of habit thing.