I did a test once where I focused on the baseboard and then made two prints. One on single weight FB and one on premium weight FB. The premium weight was noticeably thicker than the single weight but both prints were equally sharp to my eye. That said I still focus with with magnifier on a piece of paper I am using at that time. Old habit but it reduces any variables in my printing process. In theory at least a shorter lens high above the baseboard will have a greater DOF then a longer lens close to the baseboard. So a 35mm neg with a 50mm lens making a large image will have less problem with paper thickness than a 4X5 neg with a 150mm lens making a small image. But even at that I doubt it makes a noticeable difference. Maybe I will do a test with a 150mm and a small image just to see what happens.