In the SQL Anywhere 12 - documentation (http://dcx.sybase.com/index.html#1201/en/dbadmin/connect-s-3835923.html) says that:
Is there a formal (or heuristic) rule, which, depending on the "Total Time" & "Average" - values of the "Round trip simple request", "Send throughput" & "Receive throughput" - parameters tells whether to use communication compression?
No, I know of no such heuristic.
I'd guess if all of the following are true (and these are ballpark estimates):
then communication compression is likely to improve your performance. Note that I wouldn't recommend communication on a LAN. But the only way to know for sure is to try it.
Hope this helps.