The rot will only spread if it gets moisture. A little surface rot isn't a deal breaker, it's just hard to determine how bad it really is until you drag it out and look at it. I often do "autopsies" on homes where I'm repairing damage from leaks or other moisture problems. It's pretty common to see a hollowed out core between 2 studs or even up the center of a single stud. On the surface it may not look bad but sometimes you can crush it with your bare hands.
Drive a nail in it, if it's hard to pull, it may be fine. But if it seems to hit soft spots, sinks suddenly or pulls easily, there's damage that can't easily be seen.