Nope - I mix metals and I've frequently gotten the comment that it's "European." I suppose it refers to a certain ... je ne sais quoi? But, seriously ....
While I probably wouldn't wear a pair of yellow gold earrings with a white gold necklace, or vice versa, I don't see anything wrong with wearing disparate rings, or a watch of a differing metal. Life's too short to go completely matchy - take it on a case-by-case basis, and wear the things you love together. Your taste (heck, all our individual taste) is good enough to decide what works, and what doesn't!