I have an 85lb Rottie mix who is a big sweetie, but she absolutely won't let me trim her nails. She won't bite, but she won't stay still long enough for me to trim them.
I now just take her to the vet to have it done. My vet only charges $6 and my old vet only charged $7. It's definitely worth it to me to not be the cause of her stress. Also, I walk her on pavement for at least a mile a day to help keep them short.