I have never been told the reason for this but I have worked out that it is about the way the screw starts in a virgin wood hole. If you try and screw a parallel pin into a unused hole of the right size it quickly becomes very stiff and needs working in and out several times before it can be safely screwed in all the way without possibly bruising the slot. If you use a tapered screw is starts easily and only gets stiff towards the end of its travel, where it needs to be. Granted that an old screw that has been disturbed a few times usually won't time correctly when tight.
The same problem applies when inserting a new parallel screw into an old threaded hole, they can be very stiff.
A little bit of wax to lube them usually helps or I sometimes cut the required thread with a 1st tap of the right thread. This gives a good start to the thread without cutting it all the way. In a way it duplicates the effect of a tapered screw, giving a nice clear thread to run in for the first 1/3 or 1/2 and a virgin hole beyond.
I have had tapered screws made up for very special jobs but generally I use parallel screws of 3/16 Whitworth and prepare the hole as above.
Are we talking about machine shop standards or sexual experiences. "Old threaded hole" makes me think about some of Ben Franklin's rather sage advice to the young and randy Colonials- what Ho!!
