i have 08 fit and i put 112000 miles on it so far, i'm on my 3rd set tires . the dealer sayes i need to rotate them as often as 1-3000 mi. to keep them from cupping. when i look at the wheels from the back , the top of the tire tilting in and the bottom angled out. can the rear wheels be shimmed to make the tires straight up an down. the front tires never wear in any way put straight across.

 

You need to have the suspension checked to assure that you have good rubber bushings back there. Otherwise remove some of the weight in the cargo area! The rear springs could possibly be sagging, too.

Tread design, as well as alignment, can lead to tire "Cupping" or "Feathering". Our '07 (with only 55K miles and running snow tires in winter) has been fine so far...but we rotate the tires at the changeover, spring and fall.

That said, I think ~6000 miles (as the MM indicates) between rotations should be enough. The dealer is handing you a line of cr*p.

Have you ever had the suspension aligned?

 

Is the car lowered? Are you regularly hauling 2-4 people, or a lot of stuff? Do you carry a lot of junk around day to day? All of these things would cause extra loading of the rear tires.
As carbuff said, check out your suspension bushings and springs out back to see if everything's in order. After that, an alignment would be a good idea- you may need camber bolts thrown in to correct things back to spec if you're way out. If you're not lowered, that would mean that something else is worn out, though.

As for rotating, I do all of my own maintenance, so I rotate every 2,500 miles. This is excessive for most people, especially if (for whatever reason) you pay someone else to do it. 5,000 is the max I'd recommend for it on an FF. FWD tends to chew up the front tires (for the obvious physical reasons) moreso than RWD, so I recommend rotating more than on a RWD vehicle- Mark W.