I love to knit socks and those I give as gifts are always treasured by the recipients, so I really want them to last and last!
To that end I always reinforce the heels and toes on any socks with no nylon content in the yarn. A 25% or so nylon content is a pretty strong yarn so I'll sometimes skip the extra re-inforcement but even then it wouldn't hurt to use it.
Here's a great inexpensive source for reinforcing yarn that works very well. I always buy two of each color I order so that I don't run out and also because I usually knit 2 at a time on one long cable needle. (Melissa Morgan-Oakes book, 2-at-a-Time Socks)
Also, I use the slip stitch heel stitch or maybe the partridge eye stitch (Ann Budd, "Getting Started Knitting Socks") which also gives a lot of extra protection.
Re-inforcing yarn is particularly important when using a soft luxury yarn since the pressure of foot against shoe will drastically wear on the soft yarn.
Hope this helps!
PS. Jan, thanks for the Knitty article, it's super!