I would guess it depends on local practices and the size of the jobs.
From what I have been told our PMs and VPs sit down at a table with the GC and negotiate price and scope.
Items like cutting, patching, excavation we try to negotiate out of our scope. Temp lighting and power we try to negotiate to T&M. Most times we end up doing the basic temps under the contract but maintaining and adding to them is T&M.
Bob
