Indexes on a foreign key have nothing to do with referential integrity, the foreign key ensures that. If you're suggesting dropping the foreign keys, don't.
As for whether or not you should index those foreign keys, tet and see. If they help and don't measurably impact modifications, keep them. If they don't, then don't. Very few 'set in stone' rules for indexes.
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability