I have seen a number of obviously net graded coins in all sort of holders, but they say they do not do that. When the market for sight unseen bids was bigger, this was a bigger issue. Now they will details it. You don't want coins straight graded with something like a planchet flaw and have it end up on the market at a straight graded price as that affects the integrity of the entire process.
I have seen Bust dollars with horrible disfiguring adjustment marks going into straight graded holders, but other mint made issues are noted on the holder.