Something you won't find in the regs directly, but hinted at in the reply above, is keeping the imbalance to a "reasonable" value so as to not need excessive trim or (worse) control input. There are a number of reasons for this. One is crew workload in the normal circumstances, but also important is the implications for failure cases. If you're going to be considering various failures that require lateral control inputs, you can't afford to be giving up control authority to account for theoretical imbalances. And all forms of trim input - whatever the cause - are hated by the performance people, as they represent more drag, so again it makes sense to keep them small if you can without too much pain.
To the original point of a leak draining one wing, it all depends on the circumstances. If you know that range isn't an issue, you may even transfer into the leaking tank. If range is critical, you'll be burning the leaker as fast as you can and trying to save the good tank, so creating a bigger imbalance. Only when you start feeling a control issue would you stop that (and that's a lot more imbalance than any messaging) - and trying things like flying faster to gain more control power would be worth trying in those circumstances.