Coins do expire after two years. These are my leftover coins from 2008-2009. So far I've used my coins on a reversible pouch, the Mario 25th screensaver, a Mario 25th pin, and Grill-Off. I don't want to use too many of my coins because I am still saving, hoping that they add awesome things in the future. Otherwise I would go all out, but I figure either the bag or the posters would put me over but not too far over and still give me a decent item.
Edit:

The first year I registered ~1200 worth of coins, the next year I did 600, and this year I did 600. I still have more games to earn coins with, but I want Platinum status each year.
