is surface grinding worth it ?!

[diycnc]

For my build, which uses supported round rails, I just bolted them strait to the steel tube.
The whole thing is bolted together, not welded.
I checked the flatness of the steel with a length of aluminium extrusion I had lying around and a feeler gauge (it was the straightest thing have), and the steel was only out by a max of 0.15mm. The measurement was repeatable using both sides of the aluminium.

When i get around to final assembly i might shim the low spots to try and remove the small errors.

[Fancy]

Since i don't have money for a a square linear rail and grinding the square tubing wouldn't save enough money for square rails I guess i am gonna follow in @diycncs' foot steps.