Hood i know what you are saying is correct.
But i have painful memories of when i worked in a toolroom, the surface grinder J&S 540 was worn on the cross slide & i was forever complaining about it. The management naturally thought it was me, they had the service guy done who was the biggest bodger i had ever seen said it was OK.
So to prove it first i ground the mag chuck then i ground a piece of steel that was 1" wide X 6" long i ground this at 90 deg to the table
Got the management down to explain.
I mounted a DTI in the optidress ran it over the chuck perfectly level, then i put my test piece "Ground in situ" on the chuck ran it across again perfectly level .
Then turned the piece 90 degrees put the DTI on it. It read Zero +.002 Zero over the 6"
I got a brand new 540
I believe this is a common fault with all old at least J&S 540's
So this is why the OP needs to know that his axis are not curved i know he doesn't need that precision but my argument is valid
Phil