Not exactly an answer to your question.
The sides of steel tube definitely are not flat, but if machine them flat you will likely finish up with them no longer straight. There are a lot of stresses inthe tube as a result of the manufacturing process and when they are released unevenly, such as by machining material off one side, how the tube will distort is not predictable. It is predictable that it will distort; take a piece of tube and slice it lengthwise, the two pieces curl away from each a surprising amount.
It is not just the flatness of the tube that has to be considered but additional distortion from welding, or bolting, a frame together. To get flat surfaces for mounting rails you need to either do a lot of shimming and/or bed the rails in an epoxy. If you so a search using 'moglice' or 'madvac' you will find more information.
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