When knitting socks for my dad I always feel the need to make them just a wee bit sturdier than usual. Perhaps because my dad isn't exactly the most gentle and careful person I know ;) You should see him write - he almost pushes the pen through the table underneath the paper!
The reinforced heel might not be necessary, but better safe than sorry.
This is how I do it - you may know other and easier ways to go about this matter, and if that's the case do tell! :)
Anyway, instead of joining another string of yarn, I knit each row twice, so to speak. When I'm pleased with the length of the leg, I make the heel flap as follows:
RS row: *k1, sl1*.
WS row: *p1, sl1*
These two rows are repeated until the heel flap measures a couple of inches.
The slipped stitches are knitted or purled on the following row, thus making it seem each row is worked twice.
The heel loses some of its elasticity but I see this as a bit of a bonus; it gives the socks a better overall fit.
A good, solid heel flap that will last forever and ever.