Whether it is worth waiting is going to depend alot on your area. If you are in the southern US I think values stay pretty consistent. While in the north they may dip some in the winter. But if you are making a payment the dip may not be what the payment, winterization, and other stuff will be holding onto it for 6 months just to try and get a few more bucks. You may start to put together a value based on NADA, craigslist, boat trader, onlyinboards, and similar sites.
Personally I have always found there is no time like the present. Might as well give it a whirl. If you are getting beat up on price and don't like what you are hearing put it away and wait till boat show time which is usually the unofficial start to the buying season.