>In most applications you don't care the difference between a null or "" or null and 0.

Well, I do, if it is necessary. The difference to an empty date or datetime field is, that "" or 0 re regular values for their particular types, but {//} isn't. You cant do anything with an empty date value. A zero is a regular value for a numeric field. The zero tells me some value, but what does a {//} tell me?
If the zero is not valid for the special case, I would recommend using the NULL as well.

And in this special case another problem comes into the game. If you query a VFP table with date fields via ODBC, all empty date fields will show December 30th 1899 as their value. How can you distinguish between empty date fields and fields that really contain this value?