You finish the windows after they're installed. I don't understand the "concerned" comment.
Ron

It's so much easier to have the windows stained before installation. It's easier to stain them in the garage on saw horses than it is to do it inside the house and staining more than the window. The only reason most installers don't want them stained ahead of time is scratching the finish. This would mean the installer would have to be careful to not damage the finish. It doesn't bother us at all. Just keep in mind that there will probably be some touch up staining required after the installation. The only time it's easier to finish installed windows is when it's a new construction home that the painter can set up and spray the new finish on. Otherwise it's just the personal preference of the installer.