As a courtesy, I would agree upon what's going to be used, Escalation, Stronghold, Forge World, etc. and then tell them that I'll be using Dark Angels with Knight allies and rules from Stronghold and after that let their imagination go wild. Am I playing Greenwing with a whole clump of Bastions and a Knight? Am I going 3 Nehpilim with a Landing Pad backed up with a Shadowsword? That's fine, when I know I might need to expect a Lord of War or a set of Stronghold emplacements, I've been given warning and if I fail to react to it, that's on me.
But going in cold is just disrespectful if your opponent drops in a horde army and you drop in a Stormlord backed up by Leman Russ Exterminators. You give your opponent the opportunity to defend him respectfully.
Anything may be fair in love and war, but this is still a game when it boils down to it.