I swear when wrenching on my truck because there are many situations where no other word will suffice. I mean, if your wrench slips and you crush your fingers, what else are ya gonna say? "Oh darn, I smashed my poor finger?"
Swear words are here for a reason... they get across a point other words can't! I say, use them. They're only bad words anyway because a bunch of whiny-butts banded together and decided such words should offend them. When this is collocated with car repair, such language is a requirement if you hope to do the job properly. It's like a ritual I guess... not cussing when you fix your car means it won't work when you're done.