I've definitely gone (same-day) standby on later flights -- both intentionally and unintentionally! -- on UA without any problem.

Obviously, one problem to watch out for as many others have noted is full flights later on in the day.

Another thing to watch for is that while many fares allow same-day standby on any flight, there are some discount day/time restricted fares that specify that the flight you standby on must meet the day/time restriction of the fare. For example, I've been tempted before to buy a cheap mid-day fare, and go standby on the 6am flight. The cheap fare isn't valid on the 6am flight, and the standby provision of the fare says that you can't standby on that earlier flight.

I've never actually faced that rule in practice, and I don't know if they'd actually enforce the rule -- i.e., make you pay the change fee and fare difference to actually get on the other flight. But I wouldn't want to find out. (And for that matter, as others have pointed out, you'd be violating the contract if you did that. I'm definitely in the camp that says it's unethical if and only if it's a contractual no-no.)