Well you want to check your session cookies as well against something ;-)

Surely you could trust a signature and lifetime but that makes the cookie no different than a JWT (only storage wise the differ in that case).

Generally speaking it is recommended to use opaque tokens and rely on the userinfo / introspect endpoint call to make for the session management.

Only in specific cases where latency and/or scaling might become an issue you should opt for JWT. At least this is my opinion.