John coughed up some electrons that declared:


If the screw has cut itself a thread in first piece and you start with a
gap then no amount of tightening will close the gap - it may be more
likely to strip out the thread in the second piece than the first. A
clearance hole (at least as big as the outside diameter of the thread)
will absolutely assure a good clamping effect

OK - so the traditional way then. I'm happy with this. Only asked because
I've seen builders whacking in screws without pilot holes so I though it
was a "feature".

Cheers

Tim