We've just realized that if a matching address exists but it is not marked as Preferred IOM does not automatically mark it as preferred for us.  If the address is above our similarity score we never see and can't update it manually.  There doesn't seem to be any settings to cover this scenario yet so we have found a solution using virtual fields and wanted to share with others.

We added the following virtual fields to all of our imports:

Field Name: Yes             Record Type: Constituent Address Field           Value Type: Send Mail to this address             Function: Static Field

Field Name: Home          Record Type: Constituent Address Field           Value Type: Address Type                             Function: Static Field

Field Name: No              Record Type: Constituent Field                        Value Type: Is Inactive                                 Function: Static Field

Field Name: No              Record Type: Constituent Field                        Value Type: Has No Valid Address                  Function: Static Field