Not knowing anything more about your cabinets than you have described, i.e., a 1" recess, I would probably do as you described with a filler. I would feel comfortable screwing into a solid wood face frame, less so into a 1/2" particle board side panel. Use flat-head screws, drill a pilot hole and countersink and they should be unnoticeable from either top or bottom.

I finally have a couple pics of my flat panel door. Wish I had all of the doors, but alas... Anyway, in the pics, the minimal pattern does get a little lost in the walnut, but I would imagine if you did the same pattern in a lighter door it would stand out, even in photos. aimskitchen, you may also want to consider that there are two types of flat panel doors. The first truly is flat, and is usually made of mdf covered in veneers. I was actually quite surprised to see even high end cabinet lines using this kind. The other kind of flat panel, only appears to be a flat panel from the front. I believe it is actually called a recessed panel because it's sort of like a raised panel that has been flipped around. You can tell this kind because the back of the door will look something like this: Hopefully you can see in the photo that the panel is raised in the back. If you get a recessed panel you will have the look of a flat panel, but it will be solid wood and much thicker than a veneer flat panel.

Designov, I visited your post regarding your gorgeous new kitchen and due to the recessed doors that I love since I have been wanting the cabinets that you bought and the crown molding and the overall look, I felt it looked beautiful just the way it was with the matching end panels that were just not door style panels. I felt it looked pretty and finished and gorgeous. Thanks for sharing.

Do you mean shaker style doors or a truly flat slab door? If you mean a slab door then I would lean toward something sleek and modern to complement the modern look of a slab door. Drawer handles at roughly 1/3 the width are visually pleasing ... except if you have narrow drawers (like 12" wide or less) and then I go with "what does my hand fit into nicely?". I think I wound up with about a 4.5" handle on those drawers, just a smidge larger than 1/3 of the width. On drawers that are wide, like 30+", you might struggle to find a single pull that is large enough. I used 8" handles on my 30" wide drawers because the style I liked didn't come any larger - unless I used an appliance pull and those just looked wrong. It looks fine, larger would have looked better but my island has a 30" base, 24" base, 30" base in series and the drawer handles are consistent so it seems reasonable. Good luck, they are expensive little things when you need so many!