In better insoles, however, there is either a covering bonded onto the foam, or the foam is fabricated with a skin. The covering or skin provides protection against abrasive wear.
My bad! Modify the bottom surface, not the top.
(Since most insoles are flexible, modifying the bottom also reshapes the top.) Not only to protect the top layer, if there is one, but because it also avoids creating sharp edges and rough surfaces that your foot contacts. Also, in some cases it is to provide a skin-safe layer, if the bottom layer(s) of the insole aren't made of skin-safe materials. E.g., you often see a skin-protective layer if the next layer is urethane foam, which is used in a lot of professionally custom-built insoles.
AFAIK, the protective layer is always on the top - you can still sand the bottom. Do you know of any exceptions?
Likewise, if you apply tape to the top layer, then peel or sand it off because you change your mind, the tape may tear off the top layer too. So, there too, leave the top layer alone.
I don't think I have seen any hard
inflexible insoles per se in skates, where sanding the bottom won't do any good, in terms of reshaping the top - but some types of heat mold-able insole (or "orthotic" - pretty much the same thing) are thin inflexible materials, which would be hard to sand. I admit that, for inflexible materials, taping the bottom cannot alter the shape of the orthotic, but can alter the tilt. I've had bad luck with those thin heat mold-able insoles by the way - if they fit tight enough not to move around, they can push out on the side of the boots, and may gradually stretch them so as not to fit you snugly any more. (The ones I had that did that were from SuperFeet - and they denied it could happen. They claimed they weren't hard enough to stretch the boot.) If they don't fit that tight, they shift around unpredictably. But some people love them, and a lot of podiatrists use them. Perhaps they work better in high level freestyle boots than in lower level or soft dance boots.
Some very cheap skates have no removable insoles per se. And some insoles are are "tack-glued" - meaning that there is a weak glue, so you have to pull the insole out a little harder - and that might even mean that the insole doesn't fit the shoe well enough to avoid sliding around if not glued back. But I feel very strongly you shouldn't use a strong glue, even if that is true - maybe use a dot or two of hot-glue-gun glue, because you want to be able to modify or replace insoles.
Leather insoles, BTW, are quite easy to sand, and have been used in some high quality boots. Of course some leathers have a thin smooth top layer too, so, as with foam, modify the bottom.
Anyway, I'm sorry I forgot to specify modifying the bottom, and leaving the top surface be.