Swap is mainly an emergency backing, i have nagios message me whenever swap hits x amount depending on the server, it's better than oom killer picking off processes.
It might be better to use proper rlimits, and not let a mad process gone wild triggers the OOM. Swap just delays the problem.
The problem with set number limits is that changes in software and hardware require changes in limits - it's easier to just watch swap and get notified.