DisnyMik,
I usually make my reservations directly with the cruise line, just so I have this kind of control over it. I called myself, 1-877-566-0976. I just gave them my reservation # and asked if there were any openings for what I wanted (on one cruise, we wanted to upgrade to cat. 5, on another it was dining time). The call only takes a few minutes. I just made a habit of doing it when I was at work during the day. Don't even refer to the wait list, just ask if there are any openings for main seating. If not, the reservations agent should ask if you want to be placed on the wait list. I just mentioned that I already was and thanked them.
I don't know if you'll have to have your TA do this or not. I wasn't sure a TA was willing to be as persistant as I am, which is why I booked this myself instead of going with a TA (which would have been less). I know that for us, having late dinner would have had an adverse effect on our enjoyment of the cruise. Not the end of the world certainly, but I would much rather eat early.
On of my main problems with late seating is that you have to go to the evening show before dinner, at 6:30pm, so you really don't save that much time, in my opinion. You either have to get ready for dinner, or get ready for the show. In addition, I'd rather have dinner prior to the show for some reason. Just seems to make more sense to me.
jwolfe,
Main dining is at 6:00pm, and late dining at 8:30pm.