As the aforementioned FAQ describes, whenever you are selecting
cases based on their order, you should insure that that order is
unambiguous (read reproducible).
One final remark: train yourself to use -isid- with the -sort- option
instead of the -sort- command in cases where you need to establish a
reproducible order (e.g., just prior to selecting cases or
constructing an ID variable, when doing simulations, etc.). That
way, if for any reason you fail to establish a reproducible order,
you'll be sure to know about it.