Depending on your filament and filtration posture, you should probably still have some exhaust to keep the chamber at negative pressure, pulling fresh air in through the seams so fines and fumes don’t leak out instead. Yes, this does waste a little heat, but the minimum fan speed can be easily improved beyond the stock 20%. I managed to get it down to 12%. PID Controlled Exhaust Fan (Quiet!) - #8 by rpcyan
Conversely, it would be reasonable to not have any exhaust during the pre-heating phase, and only begin ventilating once filament is being extruded