Photographic paper is quite different in thickness, and what thickness do you want them to anticipate? Also, many printers like to have a sheet of paper on the easel to see and focus on. If the grain focuser assumes paper, it would double up on the paper thickness correction. What a mess!
You are better off without the paper assumption, which is likely to be wrong anyway, and add YOUR paper where needed. However, in practise, the paper thickness is only a minute part of the depth of field, and hence, this all makes no difference whatsoever.