Another app to consider (not that it makes the maps or the GPS receiver any more accurate) is
MyDirections. It actually makes Maps a usable app. (It's free, so try it. You can change it to default to car and to navigate in its settings.)
As for Maps itself, there's usually one error every so often as a copyright device. If you sell a map with that error, it's proof you violated their copyright. (It's usually something small, like a dead end street ending 100 feet further on the map than it does in reality. Garmin has some really bad "errors" - it once kept trying to take me around a parking lot - when I was going about 50 miles from that parking lot.) But I've never found Maps to be less accurate than my Garmin or Magellan GPS units, although every GPS, Garmin, Magellan, TomTom and Maps, will route you differently in some cases. The difference isn't much, but when there are 5 ways of getting there 9in the same amount of time, the unit just picks one based on whatever that manufacturer decided to do the picking on. On most of my trips, Maps has me turning left to get out of the development, but I prefer the route when I turn right, and it recalibrates within about a mile. (There's still a turn I could make about a mile down that would get me back to the route it wants. After that it routes the way I prefer. My gripe is that there's no GPS software that learns my route and uses it in the future.)
I've found Maps not getting me exactly to the spot - it'll tell me I've arrived when I'm not there yet, or it'll tell me my destination is coming up when I've already parked there - but that's within a bad GPS reading possibility. 10 meters is the accuracy with an undisturbed atmosphere and no multipath signals. (It's about best case, and telling you that you've arrived when you still have 30 feet to go is normal.) But a perfectly working GPS can be off 300 meters or more if the atmosphere is disturbed (thermals all over, or a storm), or if you're in a metropolitan canyon (like midtown Manhattan, where signals bounce 500 times before they die) or a dense forest. And that's not the GPS, that's just how the physics of the system works.