I always use reinforcement when I am knitting socks - in both the heels and the toes, as a matter of fact. And, sadly, the reason I always do this is that I have worn holes in my beautifully - and expensively - hand-knit socks! That is not a fun thing to have happen, believe me! My mother probably would know how to "darn" socks (which means repair the holes), but I don't - and besides it's not something I really care to do! The best approach, probably, is to buy yarn which already has reinforcement threads built into it. For example, I am currently knitting a very beautiful pair of socks - for my son's girlfriend, by the way - which is merino and silk! Absolutely beautiful and warm yarn - but quite soft. So it also contains some kind of polyester fibre. For that reason, of course, it would NOT be a yarn that could be felted, so you have to knit it to the right size. So, my answer is, yes, indeed, always reinforce natural fibre sock yarn or you will end up with holes in your socks!