I check them all. Just because someone says their file is good doesn't mean that it actually is... They may not verify their own files before seeding, and/or the filesets may be incomplete. Very, very rarely, I'll find something amiss. The last one was a missing file. At first glance, it looked fine (it was the alst track on a disc so I didn't notice a missing track #). But once I checked the MD5, it told me I was missing a track.
Terry