I would prefer to allow both computational as well as software questions as long as their origin is purely physical, or, in other words, as long as it can be assumed that someone completely disconnected from physics (say, in financial maths) could not answer them.
For example, numerical integration sticks up its ugly head basically everywhere and is hence better of on CompSci, but nobody who is not a physicist will even think about Feynman diagrams and ways on how to calculate them. Hence the former should be migrated on sight, whereas the latter should stay here (and made explicitly on-topic in the FAQ).
To restate the criterium:
If it is sensible to assume that someone who is not part of a physics department, or similar institution focused on physics, will encounter a given, clearly computational/tools-related, question in his day-to-day work, then the question should be migrated to CompSci (or Mathematica).