Seeing as I am an affiliate manager for one of the largest SaaS companies in the internet marketing niche I can say that you MUST have an affiliate program. It is simply the easiest way to acquire customers without massive cash outlays. Think of it as paid beta testing to continue to refine your sales funnel before you put your own cash on the line.
In addition if you don't have an affiliate program, it lends an air of illegitimacy to your product as any reputable product or organization out there has one. Given the simple affiliate management software (infusion, ultracart, 1shoppingcart, etc.) there isn't any reason not to have one. You can even only offer payment through paypal to make disbursements quick, and easy.
Let me know if you need any more help.