We really dont know. Looking at the old pictures a few of the archers seem to stand as you say, the majority 'normally'. How accurate is the artist? I do think that they would have shot without moving much. If you have a few thousand archers packed into a herce [even with staggered ranks] you are not going to be very popular dancing around when you loose, sticking the end of your bow in someones ear. I am always open to correction.........