Update and foreign key indices

Hello Folks,

I will ask my question through an example:

1) I have table PROJECT that has a foreign key USER_ID references USER(ID)

2) I *don't* use USER_ID in a where clause so don't have to create indices for performance reason.

3) USER table's primary key *never* changes.

Q: Do I have to create an index for FK USER_ID to speed up updates to USER table rows?

Basically, is PG nice enough to understand that since a key is not updated, it need not check the integrity constraints.

I am using PGSQL 7.1.3

Thanks.