Besides ...
Solomon's temple had a checkered floor
1 Kings 6:15 seems to indicate that Solomons temple had a checkered floor. This is a 'worldly' picture of it: http://anthrowiki.at/Datei:Sheba_demin.jpg
According to various sources, the Freemasons use the checkered floor because it “is a representation of the ground floor of King Solomon’s Temple”. So, if the WTS shows the temple with a checkered floor, they do that based on the Bible, and they are NOT using a masonic symbol.
Btw, I have a quite impressive collection of Watchtower pictures on my disk, and I could only find a single one showing a checkered floor - and that one picture shows Solomon's temple.