They hired you to do your best work, not to do whatever was good enough; it seems disingenuous to withhold it. On the other side of things you would have to review your contract to see if you're legally obligated or not. Now I'm not saying the thought has never crossed my mind, but beyond the morality of the issue you don't want to get a bad reputation amongst developers.

I'm wondering why you would want to hold it back in the first place. Did you sign away all rights to the music in the contract?

I've written some pretty good tunes in my time for clients but never held them back - I didn't have any reason to. Part of the business is learning to let go, and also to negotiate better up front.

Like CCH says : Unless contractually specified it's up to you to put the piece forwards for review / acceptance or not. Think of it this way, even if the game is low profile, would you not want it out there published as an example of your skill - part of your portfolio of what you can do for games. You can think of it as an investment rather than a loss.