I just bought a new set of 6600 rims and i,m fitting a new 12-25 cassette, the freehub on the rear is 10 speed only , the cassette fits nicely when locked down with no play , anyway someone told me i need to use the spacer that came with the cassette but i,m pretty sure the spacer is only for 9 speed freehubs and also it does,nt fit over the freehub anyway . The new wheel lines up with the derailer so everything seems okay , i just want to be sure .
Thanks anyway.

According to Shimano you are supposed to use it. I have the 6600 hub and the 6600 cassette and it would not shift well without the spacer. Look here:

http://bike.shimano.com/media/cycling/techdocs/en/bikecomponents/FH/SI-FH6600_v1_m56577569830563508.pdf

I'm a bit confused. What exact wheels do you have? If you have the Shimano WH-6600 wheel set, you're correct - 10-speed only, and you don't use the spacer. As Fivethumbs points out, if you have wheels built on the FH-6600 hub, they're 9/10-speed compatible, so you use the spacer with 10-speed.

I,ve got the new wh-6600 rims whitch are 10 speed only i was sure i was right but i thought i would check anyway.
Thamks again

Good luck with the wheels. I have the WH-R600s, which I've been very happy with.

I,ve upgraded the crank and wheels and i only test rode up the street and back the weather has been crap ,i can,t wait to go for a good ride .