as a best practice is to go to final flaps before you turn base, thus keeping the turn radius to a minimum
Accepting that final flaps before turning base causes significant thrust increase to offset extra drag caused by flaps extending - in turn significant triiming to offset the thrust increase, then significant thrust decrease as descent commences with more trimming etc etc. And with landing flap before base imagine the consternation if an engine failed at that point with full flap down in level flight and high power holding level flight.
One aspect of the circling approach often unheeded, is the tendency to commence descent on base in order to fit the landing profile. You should ask yourself - where is the controlling obstacle? You probably wouldn't have a clue because it isn't shown on the chart. Maybe it is on late downwind - maybe on base leg. The aerodrome chart will not necessarily display this spot height. That means once you opt to descend below the published circling MDA for whatever reason, the captain is entirely responsible for his own obstacle clearance. Easy enough by day - but by night where you cannot see the ground directly below you, a different thing altogether.