I hate to call someone a liar without having talked to them, but if what you say is true he is lying through his teeth.
Standard deployment for a general dentist is, at least for the time being, 15 months. Reservists do have 90 days deployments. I don't know how many dentists are actually in Iraq at a time, Dental Command won't even say, but I would guess that it is somewhere around 100, but that may include some Navy or Air Force dentists. 5% seems like that's the low end, but 10% might be too high.
They do not treat single dentists any different than those married with families. They are equally likely to get deployed and unless there is someone in your family near death or you have a family member with a severe medical condition that requires them to be near a hospital it will have no influence on assignments. You definitely cannot go anywhere you would like. Since the Army has fewer dentists, those slots are filled by civilian contractors, so it's not like there are open slots everywhere and you can have your choice. If you want to go to a big base, like Fort Hood, Fort Bragg, Fort Benning, you probably have a good chance of going there, if you want to go out West it will be tougher. It is true that the Army needs you more than you need the Army, but no one in charge is acting that way.
Bottom line, this is a life changing decision that you and your future wife need to make together. Unfortunately for my wife she met me after I had signed up, if she knew the last 4 years were going to be like this she probably never would have married me. If deployments continue, I would expect to be apart for at least a year during your commitment, you might get lucky and get a 6 month tour, expect to go wherever the Army sends you and make the best of it.
If it is just the deployment length that bothers you, look to the Air Force for 4-6 month tours. If that is too much, look into other repayment options, like Indian or Public Health Service. I have friends in private practice that are doing well and not having trouble paying off their loans either, and that's tuition from a private school. So consider all your options.
Good luck, I know it is a tough decision. Feel free to ask any more questions.