Bedding a rail is a lot simpler (and less destructive) than truing a receiver or lapping rings.
Every build is different, and those seeking to observe the tightest tolerances might combine all three processes. As an amateur gunsmith, and from a DIY point of view, my experience suggests the greatest gains can be found in getting the rail 'true'. With some basic tools and a dial indicator it is easily possible to achieve tolerances of +/- .001"
.0001" is a different game, and to work at this level requires much more in the way of equipment and environmental control.
Whether doing work myself, or working with others (gunsmith) I have always found value in establishing acceptable working tolerances. Generally speaking, the quality of work correlates directly with those tolerances, or more accurately the ability to quantify (measure) them.