As a background, this way of column/row labelling (or cell referencing) is called the A1 referencing style. It uses one or two letters on refer to a column (up to 256 columns) and a number (from 1 to 65536) to refer to a row.
Another cell referencing style in spreadsheets is the R1C1 style. This style uses numbers to refer to both columns and rows. It starts with the letter R, followed by a row number, then the letter C followed by a column number.
That being said, do users recognize their columns by number, or by the data they contain? do you really care that your "Total Monthly Expenses" column (for ex.) is number 40, or AN?
I think in most cases, if you wanted to reference a column, you would look for the human-readable header ("Total Monthly Expenses") that you gave to that column, then copy/use it's spreadsheet reference whether it was a number or letter. Whether you use letter vs. numbers won't impact the findability of your columns in that case.
If, however, column numbers were crucial in referring and finding columns, then I would consider the R1C1 style mentioned above.