Sanity check - thread size for linear rails

[magicniner]

Good quality M5 fixings will be up to the job.
You should never be relying upon tapped bolt holes to provide precision location unless you counter bore the two mating parts and use a cylindrical insert.

[Ger21]

When the manufacturer specs 5mm bolts, why even consider something else?

And you really think you can drill a series of 6mm holes in a perfectly straight line, with absolutely perfect spacing?