I think either approach is OK, so long as the player knows what the game expects, and that the game doesn't break the contract with the player.
Incidentally, if you are using a verb/noun game, and are using the beta version of adventuron ( adventuron.io/beta), then you can specify game_type = verb_noun at the top level of the file, and Adventuron will police your game for complex input requirements, remove standard prepositions, and will wire up a new system message for when the player types in 3 words or more. You don't need to switch this on, but it's encouraged if you are making a pure verb/noun game.