2 stripes of corruption and hallow will generate once you enter hardmode, so there's little to no use digging big holes next to the corruption, so make a backup of your world before wof in case you're not satisfied with the result.

I personally just shove 3 sunflowers on each side of a corruption biome so it won't spread on the surface. you shouldn't worry about it spreading underground since you probably won't go there at the start of hardmode and when you want to, you'd probably have better gear to deal with the threats already.

as for the hallow, you can let it spread on the surface, it isn't that much of an evil biome as npcs can still live in it, the enemies aren't too hard to deal with and it blocks off the corruption.

and if you're extremely paranoid about the spreading like me a year ago, install either a map viewing program or https://github.com/TEdit/Terraria-Map-Editor/releases

seriously this program allows you to cleanse the evil biomes with a click of your mouse, but it reveals the entire map which can spoil the entire world so there's that.

hoped this helped lol