Sorry DFC beg to disagree on that one!
Yes, there is a slight risk that when you turn the master switch on the starter solenoid may be energised and the prop might therefore turn.
However, I feel that shouting prop in this situation is like "shouting wolf" and it reduces the effectiveness of shouting "clear prop" when you are really about to start the engine. Bit like burglar alarms going off on motor calls - if it happens all the time people stop taking any notice.
At a training establishment where I was teaching a little while ago my boss (a very experienced CFS instructor) actually went and had a word with the engineers and politely asked them not to call "clear prop" when about to turn the master on and when he explained the reasons they were quite happy to comply.
In summary by all means check the prop area is clear before turning the master on but best only to call clear prop just before engine starting.
Of course, another way around all this is to have a competent member of ground crew on the outside to give you the all clear!