Suppose a server has many long-term cpu-bound processes with no realtime requirements for those processes. Would maximising the niceness of each process reduce cpu time spent on scheduling those processes?
No. The amount of time spent scheduling processes does not depend on the niceness value of those processes. It depends on the number of processes.
The niceness value affects the priority of the processes and how likely it is that a given process will be up/down prioritized in the run queue.
It is very uncommon that the time spent scheduling processes is ever an issue though.