Ewe: I would definitely clip one of the  before going for a visit to the vet.  Two of them are perfect patients, but one is not.  She's strong, and cut the doc pretty badly when being given an injection.  I'll also trim their dew claws, which can come pretty close to growing round in a circle.  Might trim them if I see them chewing on their nails, too.

But lately, there's been less need for trimming, because of a happy accident.  They finally stopped shredding my nice furniture almost completely once I brought home a rug bought on sale from Ikea.  This certainly wasn't my plan, but all three of them love sinking their claws into it--especially the underside of it for some reason.  It gets 99% of their scratching attention, and I will happily sacrifice another rug just like it next time I find them on sale.  Isn't that the craziest thing?  I keep it rolled up, with the underside out--and they're absolutely loving it to pieces...