Just to be anal about this, the speedo error out of the factory shouldn't be discussed in mph terms, but in % terms. A 3mph error at 35mph is serious, at 85, it's a lot more trivial. And as I said previously, there is so much variability in tire size (actual vs. advertised, and new tire vs. worn out tire) that it's always a good idea to measure the error instead of assuming. There's also the possibility that a previous owner had changed something, or that your particular speedo has other accuracy issues. Speeding tickets are expensive, checking is easy. And on the other side of the same coin, you don't want to be driving around a few mph under the speed limit when you really want to be going the speed limit.
Bottom line, I can't imagine why someone wouldn't want to know how fast they're really going if it's cheap (free) and easy (trivial) to do so. Guesstimating is fine for a starting point, but when you're coming up behind that cop in the right lane, don't you want to know if you're going to be going the speed limit, or 2mph over or 2mph under if you pass him? I know if my speedo tells me I'm going 75mph, and the speed limit is 75mph, I can cruise on by him safely. If you don't know, you can get stuck behind him for 20 miles doing 72, or maybe you can get a ticket for doing 77mph if he's in a bad mood or behind in his quota.