iStock: doesn't matter; system will eventually rearrange them into most relevant anyway according to views and sales. At least that's what's supposed to happen.
Alamy: matters a lot (in theory) because of their time consuming system of essential keywords, main keywords, comprehensive keywords, but in fact looking at my own measures (stats) I can see keywords mixed and matched from any and all sections, including the caption and the locations. (NB, the description is not searchable.)
No idea about the others, sorry.