A valve clearance check takes me 5 hours total. It may take a shop 3-4. If shims need to be changed that takes me 2-3 hours. It may take a shop 1-2 hours. Is it necessary? For me it was. At my first valve check at 16,000 miles I needed to change one shim as the valve clearance was too tight. If I didn't do the valve check, shim change I'm not sure what would have happened. Maybe the clearance would have been so low that after a while the exhaust valve wouldn't have closed fully during the compression stroke?
I would do it. I second the recommendation to have the shop give you the clearance measurements so you will know how they've changed on your 32,000 mile valve clearance check.
If you're a risk taker, there was a poll done on this site a while ago that found only about 20 percent of owners needed to do a shim change on their first valve clearance check...