I knit socks using 5 dpns all the time. I divide the instep stitches over two needles and the heel stitches over two needles and knit with the fifth needle. It works great because I don't put as much stress at the corners where the needles meet. I usually divide so that there is a beginning of the pattern on each needle. Sometimes that means an uneven number of stitches--Say the whole sock has 72 stitches and a pattern repeat of 4 stitches. I would have my needles as follows: Needle 1:20 stitches
Needle 2: 16 stitches
Needle 3: 20 stitches
Needle 4: 16 stitches
Needles 1 and 2 eventually become the heel/sole
Needles 3 and 4 the instep
I tend to not think about the needle numbers as much as I'll think instep and sole.