Really depends where you live. Usually 10w is the standard, but if you're in an extremely cold winter environment, going thinner (5w) can help with the extreme cold weather. I'm in Chicago, and 10w is fine, though we'll generally have a few of those extremely cold days every year where you can tell the oil takes a bit to circulate through the engine.
If you're going to see a lot of 15 degrees or lower, go with 5w. I don't expect you'd have a serious problem either way though, so I wouldn't sweat it.