To Zero or not to Zero

This is more of a design question about how data should be stored in a table.
While importing information from Excel, some records in the same field have
zeros and others don't, meaning they are empty. All the fields in question
are numeric. So I am looking at an update query to clean up the table and I
have two choices. I can enter a null value if it equals zero or a zero value
if it equals null. The zero value will make formulas easier, meaning the nz
won't be needed, but the size of the table will increase (OK, maybe not that
much). On the other hand, the fields are like credits and debits, for each
record you will get one number and the other will always be nothing, so the
zero value doesn't really mean anything to me. So my question is this, what
question should I be asking about the data to determine whether I store a
zero or null value in a numeric field?

As always, thanks in advance for any help
--
Michael Conroy
Stamford, CT