Null?

One of the biggest questions debated by Database Administrators is over whether or not to allow NULLs. Right now, TPAS has many fields that allow NULLs. I've ran into a few problems with this but managed to fix them. My question, should the TPAS database allow NULLs? One reason for eliminating NULLs is the uncertainty in the way different databases and PHP database drivers handle NULLs.