Bird baths should be placed in the shade. There are several reasons for this. Firstly, many birds use bird baths to cool off in the heat, and the shade will keep the water in the bird bath cooler for longer. Keeping the water cool also means that less amount will evaporate and, hence, the basin will require refills less often.
Algae can grow rapidly in a basin of water, leading you to need to clean it more often. Compared to being in the sun, algae production is slower in the shade.