[name]Don[/name]'t worry about it! I know a girl who goes exclusively by her initials MT, and nobody has given it a second thought. By all means, use MT.
To be honest with you, when I read the title of the post I thought you were concerned with the fact that MT stands for mountain, not that it sounded like “empty” and I didn’t even pick up on it til I read the whole post. Either way, I don’t really see it as an issue. It’s one of those things that kids may or may not pick up on. Especially if there is a child with more unfourtnate intials,i.e. PIG, [name]CAT[/name], WIMP, etc., MT isn’t really a problem. And the kids that may pick on “mountain” or “empty” will drop it shortly later due to the fact that it’s not very interesting or all that “mean”. I say go ahead and use the names you like and not worry about the intials possibly being MT.