From a straight performance perspective:

The rule of thumb is that DBs are bound on IO, so any calculation that you can get with the same amount of IO as returning the records is likely to be "free", and any calculation that requires more IO is likely to be "expensive" (unless that allows you to avoid IO by restricting returned records, avoiding future queries, etc. - it gets complicated fast).

So things like column-column calculations, simple aggregates, etc. are likely to be good ideas on the DB; for anything else It Depends.