I agree with Willis, put the splice 4' above the floor to allow the contractor to use the columns for tie-offs. I would include a splice detail. Personally, I wouldn't note the splices as "optional", but I would be open to the contractor requesting to provide long columns instead of the splice and alternate splice details if requested by the steel fabricator. It may be a cost wash between deleting splices and adding steel weight at the higher levels where it is not required.