Ella Kelly asks a great follow up question to my last post...
She describes herself as a yarn snob - she works with all natural fibres, all the time. She's just bought some 100% wool yarn for socks. The yarn shop recommended that she add nylon reinforcing thread to the socks, and she asks if I do that myself when I'm working with 100% wool yarn for socks.
The answer is a definitive "sometimes". It depends entirely on how I'm going to use the socks. Are they for boots? Will you wear them a lot, in active situations? Or are they for mostly wearing around the house in slippers, or while resting on the couch, or in your work shoes while your feet rest quietly under your desk?
In short, if they're going to take a lot of wear, yes, do reinforce. Reinforce both the heel flap and the heel turn itself, and I also suggest reinforcing the toe and the last inch or so of the foot before the toe.