As early as December 2009, the FTC released guidelines that require any affiliate who promotes products to clearly disclose the fact that they receive compensation for doing so. So, yes, you do need one. It is the law.

However, policing and enforcing this law is obviously an issue, so you probably don’t have much to fear if you don’t have one. However, I believe it is better to be transparent and honest. There are creative ways to include disclosures and I don’t think customers with whom you have built up a trusting relationship will have any issues about you receiving some compensation for your efforts.

This tutorial includes:
What is an affiliate disclosure?
What should it look like?
Where should it be placed?
Sample disclosures
How do you include it?