If you have one or more field nullable, VFP add hidden field (1 byte lenght) to the record structure.
Then use null use one byte ( space is very small ) and one field ( this is more restrictive ).

But, you don't hesitate: use NULL, any other choice it is mistaken.

A little ex.:

CLEAR
CREATE CURSOR C1 (F1 i NOT null,f2 m NOT NULL)
FOR K=1 TO 10000
APPEND BLANK
NEXT
? RECSIZE(),HEADER()+RECCOUNT()*RECSIZE()

CREATE CURSOR C1 (F1 i null,f2 m NOT NULL)
FOR K=1 TO 10000
APPEND BLANK
NEXT
? RECSIZE(),HEADER()+RECCOUNT()*RECSIZE()

CREATE CURSOR C1 (F1 i null,f2 m NULL)
FOR K=1 TO 10000
APPEND BLANK
NEXT
? RECSIZE(),HEADER()+RECCOUNT()*RECSIZE()

CREATE CURSOR C1 (F1 i null,f2 m NULL DEFAULT NULL)
FOR K=1 TO 10000
APPEND BLANK
NEXT
? RECSIZE(),HEADER()+RECCOUNT()*RECSIZE()

CREATE CURSOR C1 (F1 i NULL  DEFAULT NULL,f2 m NULL DEFAULT NULL)
FOR K=1 TO 10000
APPEND BLANK
NEXT
? RECSIZE(),HEADER()+RECCOUNT()*RECSIZE()

>My app will connect to either a VFP database or a SQL server (switchable by changing an INI file). I have set the TEXT field in SQL, mapping to a MEMO field in VFP, "not nullable" and default to an empty string. I think this is a better choice over setting the TEXT field nullable and default to NULL since code for both VFP and SQL backend will be easier to be kept the same.
>
>What I understand is a TEXT field is a pointer to actual information. May question is will defaulting a TEXT field to an empty string (real value) take up more space than defaulting it to Null (no value).
>
>TIA