If OP has access to the non-rev site for legitimate reasons, I see no reason why he shouldn't use it to glean operational information. From a terms of service perspective, no one can definitely say that OP wasn't legitimately contemplating booking a non-rev ticket on the flight he's searching up. Even if he had already purchased a revenue ticket, he could have every intention to cancel that revenue ticket (e.g. if he notices that there seem to be lots of empty spots).