As always with Oracle, the answer is "it depends".
1) You should always question the logic of updating any indexed or constrained column unless it is absolutely necessary. Why? because you will force a lot of extra work that is unnecessary. You should only update a key column if the value has actually changed. This is a flaw in the "update the whole row all the time" approach that is common to many applications using "popular" frameworks.
2) Foreign keys are usually used in join conditions when querying the database. With this in mind, why would you consider leaving an FK column unindexed?
3) If the number of updates that could cause this issue are sufficiently small, and not in vital functional areas, do you care about a little extra locking?
4) In 9i the impact of this locking was drastically reduced. See: http://www.oracle-base.com/articles/9i/ ... indexedFKs