"iscachable" only affects how often the function is called in an individual query (as in "once per unique set of parameters") so it's safe to use on any function that only depends on its input parameters (i.e. doesn't SELECT from the db or call uncachable functions etc).
So, yes, feel free to commit.
BTW there are more extensive cache options in 7.3 I think - at least they were discussed. Maybe not 'til 7.4.
