I think all the online mapping sites use Google Maps for their data. It has to estimate the actual slope of a hill based on a somewhat small set of known elevation points. So sometimes, on the 3-D Google Earth, you'll see ponds wrapped over the calculated edge of a hill, or a parking lot at a 30 degree tilt near a cliff.

So the mapping sites are good for general elevation, but you may find a short, steep pitch that doesn't show up on the maps.

mapmyride tries to filter out the small dips and bumps on any route, so it tries to not count the little elevation changes toward the total elevation gain. But it mostly skips any elevation change less than 10 meters/33 feet. It does have a downloadable table of the elevations every 1/100 mile or so, which might help you.