If the other workbooks are not of an earlier version, I would ignore the message.

The times when I get similar messages is when the extension is .xls If all other versions of Excel in use are 2007 or later, convert your files to the .xlsx extension by opening and saving in that format. You may have to re-link where links are used but it could be well worth the time and effort.

I think that this compatibility thing is enough at times to make you go bald pulling out your hair.