There is a spesific date that the grand list for a given year becomes effective. One day later and you are not on that  years grand list.  Depending on what the tax bill is and other concerns it may be worth it. 

Eric

I can only answer this question for my state (KS), so take this for what it's worth.

In KS, January 1 is the date on which the property is valued to determine property taxes for the year (this seems to be consistent with your builder's explanation).  However, if a house is partially completed on January 1, the county assessors generally assess the value of the home on a percentage completed basis.  So, if the house would have been worth $100,000 on Jan 1 if completed, but is only 50 percent complete, then the value of the improvement (not including land) would be $50,000.  It is possible that FL has a statute that does not value the improvement (i.e. the house) until it is complete.  I haven't heard of any state valuing properties for property tax purposes that way, but it's possible.