Nick Usborne recently wrote an article called "Where and how to place your links in a 2-column HTML email".

These are the tips he offers:

1. Links in the side columns will not perform as well as those in the center column, even if they are accompanied by nice product photos.
2. In the main column a simple best practice is to place a text link early on in the text as well as at the end.
3. When writing the text links, a descriptive link will usually out-perform a shorter, instructional link. In other words, “save 20% on our end-of-season Sea Kayaks” will likely do better than “buy now” or even “check out our low prices”.
4. If you are selling more than one product in your email, you will probably get a higher click-through on the first product mentioned, and the lowest click-through on the last. (This is generally true, but definitely not true all of the time.)

He does add that "the best practices I describe are true of what I have seen overall. That said, I also see metrics on emails that fly in the face of all these suggestions."

So you should still test what works best for your audience.