Only once, ever. My Mac Pro started kernel panicing not long after start up, which the logs were indicating was a problem with the bluetooth driver. A 'repair permissions' detected a problem somewhere in /Library and fixed it, it's been fine since.
The odd thing was that it didn't kernel panic after every boot, more like 80% of the time, which made me skeptical towards the theory it could be a permissions thing, since they don't change at random, however it did fix the problem.
I'd never 'repair permissions' routinely though, what's the point?