If you have to force deflection it's probably too tight. I like the slack in my chain to feel a touch floppy, not taut like a bowstring, so when you deflect it you are actually just lifting it up a bit, fighting gravity not chain tension. That way you're sure it's ok. There's really no safety related reason to have it as tight as possible without binding and it's too easy to get it too tight and mess up your bearings and drivetrain.