When doing heels I do two things to help keep the holes minimal (ok, three ...)
1) do the heel with the yarn tensioner in use, and make sure there isn't slack in the yarn when you start each row
2) on the very first time across (and back), wrap the outermost out of work needle from the top of the foot before starting across (and then before starting back). This extra wrap helps close the "O" that tends to form right at the start of the heel
3) lower the last needle knit (first needle to be knit) and ensure that the yarn wraps the latch on its needle as it knits that first needle coming into play
With these techniques, the only time I get gaps are when I knit at too loose a tension (and the overall sock tends to be filmy then too) or when I put down/lift up too many needles at once (i.e.,a mistake). ok, a third way -- when stitches in the heel drop and I don't recover them correctly!
(posted by me on sockknittingmachines this day)