If you remember a LOS is clear from a previous game, well, then you remember that. Good for you. Nobody has control over that. Me, I have trouble remembering LOS is clear from a previous turn.
Deliberately checking whether a LOS is clear or not when the rules specify that you're not allowed to do that is not, however, "personal circumstance" or "convenience" or even "silly". There's a different word for that: it's called "cheating". You should not do that. Your opponents should not do that. If you were my regular opponent and you told me you were doing that, I would ask you to stop. If you then indicated that you had no intention of stopping, you would no longer be my regular opponent. It's not rocket science. Only check LOS when the rules allow you to do so.