My DM does this for immediate actions in her regular 4e campaign. If you declare an immediate action, but it turns out you can't actually use it or don't want to use it, you'v used up your immediate action until it re-sets at the end of your next turn. It has helped speed play now that we're in Paragon levels...as your DM, if you don't know exactly what maneuver you're going to use as your reaction, or combined with your standard action, I may take it away from you for that round to speed up the game.
What do you think about that? Am I too harsh?