The 'always assume it's loaded even if it's not' crap is essentially saying that you've not effectively cleared your gun, or you don't trust yourself sufficiently to trust your own eyes to yourself.
Certainly, the whole 'trigger discipline' thing has it's time and place, when stacking up on a door with real firearms for example it's probably pretty damn important that you don't shoot your mate in the back. But by and large, very very few british airsofters are ever going to need to do that and so 'treat it like it's real' is really kinda pretentious and a bit sad.
That said, like Samurai I don't take my eyepro off even in the safe zone. It only takes one idiot who didn't clear his chamber after removing his mag and someone's in for a shit day, guess what though, that's not gonna be the guy who has any care in the world about 'trigger discipline'.
p.s. fuck off, trigger discipline. just fuck off.