I would say it is up to you, but I would not recommend it in my opinion.
The main considerations are:
- All that the FKs will do is enforce referential integrity (they will not impact performance as much as carefully managing your indexes).
- As far as I am aware, there are no scenarios within SimpleMembership that will break if you use referential integrity.
- But as SimpleMembership creates the tables without the FKs, that is the way it has been tested to work; creating the FKs may therefore break some operations (however unlikely that is).
Therefore I don't create the FKs. I use the SimpleMembership, WebSecurity, Membership, Roles etc. classes to do all membership operations, meaning I never need to access the tables directly. This also means that if there are edge cases where the referential integrity would break the SimpleMembership provider, I am not at risk of coming across them.