It's really the same process, one just documents the errors, the other fixes them. You certainly don't have to verify first. Verification is useful mostly when people have custom permissions, that they don't want "repaired"... they can get a list of files that *do* need a repair this way.
edit: you can only repair permissions on volumes that contain OS X. So just do your boot volume. Some people recommend repairing permissions after any installations, or once a month... I run it whenever I have a problem, no more. It's certainly something you should learn how to do, for maintenance or troubleshooting...
paul