As Steve said, you can use regular papers for contact printing. No problem. I did that myself for a number of years. But if you want the best prints that can be made from your negatives, you will want to print them on Azo. Why? Better tonal range. Deeper blacks. More glow. More "presence." Outside of that, I guess there is nothing else. Oh, yes, one more thing. Contact printing on Azo is a lot easier than contact printing on enlarging paper. Because of the long tonal scale of Azo, there is only about 20% of the dodging and burning required than is required on other papers. And as a result, printing on Azo will enable you to print negatives that are otherwise unprintable. I could go on and on. I have already: see www.michaelandpaula.com. Look under "Azo" for my articles and those of others and for the "Azo" forum.
Michael A. Smith