In the early days I did because the affiliate companies provided them automatically, however I don't use them now. Instead I track sales from a certain affiliate link and compare it to the sales of other related affiliate links, plus compare their location on the page and the content surrounding them. Might sound old school and slow and it possibly is, but it seems to work best for me.
For a list of affiliate links, I'll track which of the 10 links for instance is earning the most, so if link # 6 is earning twice as much as #1, then I'll bump #6 up to the #1 position and double or triple the income from that one specific link. This only works of course if you have them in list form under a title such as "My favorite Freebies", "My favorite Income Sites", or "Check out these Great Hosting Companies", etc.
An interesting question and a good one - thank you! :) Whit