Im not sure as far as an official rule goes, but i guess everyone is saying, you have to tell. but lets look at a few things:
showing someone/newbie/friendly game - show them, remind them, be very clear. dont dick over a new player when you are trying to sucker... er. get them into the warhammer universe, its just rude and unfriendly
rival match/playing a regular - i think you can both put aside your egos, or manhood aside and agree on a rule. you wanna be secretive, so be it, if not, so be it, but agree from the beginning of the game, and make sure there is no accusations, cheating, the lot..
tournament - play by the rules, be VERY clear, mention where you stuff is from the beginning
'ard boyz tourny - don't ask, dont tell.
but seriously, comes down to dont be that guy nobody wants to play.