MapMyRun is distance-accurate AFAIK. There are other options (e.g., www.gmap-pedometer.com, http://www.plotaroute.com) but MapMyRun is the one I usually use to map new routes.
I have found it to be hit or miss with accuracy. Locally, mine is pretty spot on but when I have traveled and run other places, it's off. For example, it clocked the Miami Half Marathon route at 13.8 miles rather than 13.1.
Map My Run should be fairly accurate. The distance will always be slightly off due to slight weaving and maybe not running the turns and corners most efficiently, but that wouldn't create a huge difference.