Text vs. number

I was under the impression that a field should always be formatted as text
if the contents were not going to be used in calculations, so I formatted
fields such as Social Security numbers and employee numbers as text. But I
notice many examples in the NGs using numbers for this type of data.

Should I change the formatting? Does it matter? One issue that comes to
mind with formatting as text is that someone could enter a letter into the
SSN field. Any other reasons?

Thank you for any suggestions.