if we file at the SOS to cover the risk that the "fixtures" are later classified as equipment
That too, but fixtures are filed in two locations:
1) in real estate records (that is a fixture filing) to prevail against those with an interest of record in the real property or are encumbrancers of real property; and
2) with the central filing officer (you stated SOS, I guess that is the central filing officer in your state) for all others with a security interest in FIXTURES.
Read 9-501 carefully and you will see these two filings.