It doesn't change much year-to-year, although spring can be a bit worse than the fall after the improvements are done for the season. The highway isn't open or maintained in winter, so in spring there's lots to do to get it ready for the summer season.
The Alaska side (Taylor Hwy) tends to be worse. The Yukon side (Top of the World) runs along rocky ridges without rivers/marsh/muskeg so is usually in better condition.
We drove through in 2010 after a major washout on the Alaska side and lengthy closure while it was rebuilt. No problems towing our 25' fifth wheel through with the F-250, but we weren't in a hurry and usually plan two days for the trip between Tok & Dawson City.
Taylor Hwy (Alaska side):
http://yukonsights.ca/TaylorHwy.htmlToW Hwy (Yukon side):
http://yukonsights.ca/TopOfTheWorld.html