Always normalize as far as necessary to remove database integrity issues (i.e. potential duplicated or missing data).
Even if there were performance gains from denormalizing (which is usually not the case), the price of losing data integrity is too high to justify.
Just ask anyone who has had to work on fixing all the obscure issues from a legacy database whether they would prefer good data or insignificant (if any) speed increases.
Also, as mentioned by John - if you do end up needing denormalised data (for speed/reporting/etc) then create it in a separate table, preserving the raw data.
