I think filing negative carriers is something that one regrets sometime later, like getting a tattoo.
I agree. I could never bring myself to do that.
Cut a piece of board a couple of mm - or however wide you want the border - longer and wider than the exact size of your print, black if possible including the cut edges (blacken them with felt pen). Expose your print in the easel. Lay the board on top. Open the easel a couple of milimetres all round. Align the board top and left with easel, so that there's a newly exposed bit of paper to the right and bottom. Expose to full black. Move the card to the right and bottom and repeat, thus exposing top and left of print. You can also leave the card in the middle but I find that less reliable. Use weights to keep the card flat and stable.
This works for crops aswell, you don't need the whole neg.
Or get a glass neg carrier -e.g. for 6x7 and use that, you can then get the whole rebate if you want, which is fun sometimes.
hcb also liked to show that he did not crop his negs, a bit of a wank but it is good practice to crop in camera not on the neg
Problem there is that it forces you to crop to the camera format rather than the subject. If I am shooting 35mm and my subject matter composes better to a 3:4 or square or some other completely random aspect ratio, I don't hesitate use the bands of my easle. Better to let the best composition ask for the best dimensions than to try to "wank" it into a standard predetermined format.
(Even if it means that you you have to custom cut your mats for each print. What a pita!)
That is called grain. It is supposed to be there.