How To Prevent Holes Around The Inner-tube Valve?
I had a spate of these; all the tubes had an elliptical, smooth rubber section immediately around the valve, and then a weld joining this to the body of the tube. The failure was always at this weld, and it looked as if the tube had herniated (bulged out in a single spot before bursting there).

After ruling out tape problems, burrs, sharps, valve hole diameter etc. - I finally figured out the problem.

The elliptical section is less flexible than the main body of the tube, so after pushing it above the bead as Zippy suggests, it wasn't fully descending to sit flush against the rim. This left a void between the rim and the less-flexible part of the tube around the valve; the more flexible section of the rim then tried to expand around the weld to fill this gap, and that's where it burst.

The solution is (touch wood) to pull the valve back down after the tyre is seated and the tube partially inflated, and tighten the nut more than usual (and then slacken off).  When the tyre is fully inflated, double-check that there is no bump near the valve, that the bead is seated properly there, and that the valve doesn't move if you tug it towards the hub (not too hard!).