No, not really - any symbol would do, provided you made it clear somewhere that it represented a variable - after all, most people would understand the problem "Factorize ¿² + 6¿ + 5 = 0". But you'll need to find a way to represent them that you can generate easily on a keyboard - and that's composed largely of alphabetic characters - so is there a good reason NOT to want to represent them that way?
