While it is not absolutely essential, it will benefit you greatly if the affiliate program provides detailed statistics on your sales and referral activity. And the more detail, the better. Two reporting features to look out for are referring pages stats and the ability to add your own ad-tracking codes to links.
As an affiliate, monitoring/tracking sales activity is an essential part of the day-to-day activities. While affiliate programs should at a minimum provide basic reports on impressions, click activity and sales or leads, there are a few whose reporting systems are lackluster in this regard. If the merchant cannot supply even the basic reporting, it’s difficult to be confident that anything is correctly logged.