Don't EVER cash them.
You need most of this Souls to buy Magic, Miracles or to upgrade your weapons into special versions of them.
The thing here is, though, that you need to finish he full game 4 times if you want the Platinum Trophy. Im still working on that. I've already acquiered all Magic. I need about 3 more miracles and a few weapons... which means I need 2 more games.
I guess you can cash them then, but it ain't worth it.
Im level 159 btw.
Any of the DLC characters available already? Or rather... is Makoto available?... since Platinum and Valkenhayn are still on development.