There should be no harm in attaching the marriage certificate with your wife's application. TSC should have a copy of it in the I-485 file, but apparently they do not look in the physical file when processing the EAD applications.
They will try to find a matching record on the I-485 filing in their computer systems, which reportedly are based on the Last Name?! Therefore make sure you use the same spelling of the last names for you and your wife as the ones used in the original I-485 filing.
If they can not find a matching I-485 filing for your wife, they might need a marriage certificate to prove the relationship.
Also for EAD renewals it helps to attach the page (sheet), received with the previous EAD card (if you still have it) - it contains some special information, which helps TSC to verify the renewal and expedites the whole process.