Cindy- I think it depends on what formation your opponents are in. If your opponents are 1u1b then if your partner hits then I would probably move up as soon as I saw the ball go to my opponent- I could be in position no matter what and there isn't a good reason to be 2 back against 1u1b.
If my opponents are 2 up then I will stay back until my partner hits a ball that they think they can move up on. But that really depends on how effective my opponents are 2 up. I won't stay 2 back if my opponents have weak overheads.