The first tool many DIY mechanics grab when engine performance seems to suffer is a compression tester.  While the compression tester serves it's purpose of determining what cylinder may be damaged, it will never pinpoint the actual problem.  A bent valve, damaged piston, head gasket or warn piston rings can all contribute to a low compression reading.

I feel a compression test should be used as a tool to compare before and after results.  Record your readings after a fresh rebuild and compare them to readings at the end of the year or after an event.  Keep in mind that engine temperature, cranking RPM, compression ratio, cam lift/duration, cam timing and the duration of the test can all change the final compression test value.  This is why it comparing one engine to another is not recommended.