Now maybe she meant just whether a whisper was necessary or not. If a player is marked LFG, or they're a guildie you know is available for a group, or they've just shouted out "LFG for group quests" in a chat channel, then I'd say they're fair game for a ninja invite. You don't need to actually whisper them and confirm everything you do (well, it's still nice to do, but in those cases, I'd say you don't need to).
But yes, invites out of the blue are a no-no. Rude, maybe, annoying, yes, and usually a waste of time for both parties.