Originally Posted by Mike, K8LH

Careful Peter, you're getting some sloppy advice.

If you're scanning columns (only one column is on at any one time) then you'll use current limiting resistors on the row drivers. If you were to put current limiting resistors on the column lines when you're scanning columns you'd get a different voltage drop across each resistor depending on how many LEDs are lighted in that column. Your row drivers are only ever driving a single LED at a time as you scan columns.