Yes it's definitely beneficial to know if the vales aren't sealing properly, but not while you are CC'ing chambers or ports for that matter. Found that we normally do CC'ing before and while we do flowing in the ports and chambers, but before we do the seats.

This is because you sometimes just nick the seat when busy with the porting and flowing, which will cause enough water to drain to make the measurements inaccurate. Also on old heads that you want to take preliminary measurements on, the seats could be pitted and not sealing properly. At this point it doesn't matter if the valves don't seal 100% yet, because they are still to be cut. So it's useful to use a thicker type fluid that doesn't drain away very quickly through the smallest of hole/s.

Once that kind of work is complete, you do the seats then we normally use paraffin(kerosene for US) or diesel as a final measurement. It's thin enough to show any leaks the vacuum test might not have picked up, but it won't cause your pristine seats and valves to get rusted.