It depends. I would say it's only worth it if you remove the stock foam, then you can truly tweak these headphones to different sound profiles, from very flat to super bassy, probably even more than EQ could (but I'm not 100% sure about that, maybe EQ won't distort if you tweak it in the same way how some foams affect these headphones).

I personally like it without foam, but because I'm worried of getting dust into my 6XX I added stretched pieces of nylon between the housing and the copper rings, basically a dust protector that doesn't influence sound.

With some inserts you get a close to stock 6XX, with others you get a very different experience. Dense foams give you super flat and analytical sound, really not what I prefer. Anything with holes in it or lower density is pretty nice.

So if it's worth it is up to you. It's fun to mess around with it, but a stock 6XX is just blissful so it's not like you're missing out. If you like bass, it's worth it. Watch the YouTube video from custom cans to get the measurements I'd say and then decide.