If the tube wall is about the same as the screw diameter then threading the tube will be fine. But, I don't see anything wrong with using rivet nuts.
I'm about ready to build a plasma table, and will be using 1/4 (~6mm) wall tubing, I plan on tapping the mounting holes for both the rails and racks. But I will be drilling/tapping on a CNC mill, so will be pretty easy for me. If I had to do it by hand I might rethink the process.