I know this is a broad question with many reason to do and not to restock. Resale value,maker,grade etc. The reason I ask is several times over the last year I've come across a other wise nice SxS but it has stock issues? Cracked,poor repair etc. My thinking was they maybe a gun worth restocking to fit me and use forskeet or trap shooting etc. currently looking at a nice LC Smith field grade 20 ga. Only problem is it has a crack and repaired stock. Everything else about it is solid.
If you are a frequent seller of guns, or, have a thought of selling the restocked gun at some point, almost never. I've had some restocked, still have one (I'm lefthanded, and it is good to have a lefthanded gun that fits) but, as a rule of thumb, the only person who cares for a gun restocked to fit you, is, well, you.
I don't know what will happen to, or how much money, my lefthanded restock will bring when my heirs are sorting through the debris of my hunting/shooting career. I guess I don't care, either.
Best,
Ted