Do you guys change your aspect ratio to 4:3 or 3:2 when shooting stills? Is there any benefit to shooting in those vs 16:9? Obviously using 16:9 would be good if the stills will be incorporated in an HD video, but what if that's not the destination for them?

I leave mine it 16x9.

You would probably want to consider the final destination and purpose of your photos when deciding this.

If your photos are going to be used for a wedding photo album, or if they're going to be printed, you might want them shot in the traditional aspect ratio, but I cannot think of any other reason.

For printing a photo for use in decorating a studio, the 16:9 might look cool, but I don't know if places that print photos have paper that can do that size, or even how that works, so that would another consideration, I suppose.