No fixed "rules" ... on this occasion I had a couple of ideas in mind, but it became obvious after the shoot that they just weren't going to work the way I'd hoped -- and there was nothing I could do. So rather than flog a dead horse I just dropped the folder in the bin and went back to posting on CiC and crashing RC helicopters!
For a normal landscape shoot I usually end up with quite a few of the same composition - with progressively longer exposures, so I normally just pick the best one and post-process that. After a studio shoot I usually only delete the blinkers and OOF shots - then let the model choose the ones they want for further processing.
I guess if there was to be a rule of thumb then it would probably by "if it's beyond redemption then delete", but having said that, it also takes a lot of experience to judge whether some shots truly are beyond redemption.