Chipped spline on 8.75 axle...let it go or change it???

I would do this just so there are no sharp edges.
Yes, except that the spine was hit on one side and might have deformed the adjacent side of the spline. If it is deformed it will cause interference. I would make sure that the side adjacent to the chip is not deformed, if it is it should filed flat or so that it doesn't cause a problem.