On rails they should be at 90 degree increments. And when the minecart is moving, this data is correct; however, when it starts to get too slow, the data stops updating. 

In the attached images, I get the Rotation of the Minecart at several points along the track, showing that it has correct data. But at some point it stops updating the Rotation NBT, notably during a turn. And even if it continues moving and rotates even further, it does not update again. The rotation nbt updates when the Minecart speeds up again.

Visually the Minecart continues rotating, it's just the Data that doesn't update