Me personally, I hate it when elaborate NumberFormats obfuscate the data that's really in cells - to me it makes troubleshooting a lot harder, but I'm not sure it would affect your pivottable sums and such things anyway. But, that's just me, some people use a lot of weird number formats and love it.
It depends on WHAT actually IS in the cells that you are calling "Null". Are you exporting this data into Excel from somewhere...like SQL server where they actually paste as the word Null?