My advice? If you want it, get it now. Installing one later will be an expensive pain in the neck - there are just too many things that can go wrong when doing something like this after the fact - lining up the trim, not damaging the counter, centering the window, etc. That said, and this is just my opinion, garden windows are a pain in the neck, anyway. Who wants to climb over the kitchen sink to tend plants and clean glass?

John

It depends upon the type of installation you are contemplating.
Have the countertop fabricator leave you additional material so that you have the option to create a window sill, if you so choose.