Pieces of metal can get caught between the piston and the cylinder and make some marks which may or may not cause a significant increase in oil consumption and often a measurable loss of compression. Back in my earliest days of working on cars (I don't think I was even a teenager yet), my dad showed me how one could handle the problem with Fords and their then new huge 18mm spark plug holes in cast iron heads: bring the piston near top center on the firing stroke, stuff a coil of clothesline coated with sticky grease into the cylinder, cut the threads (or do the helicoil installation) and then use a tweezers to get the clothesline out. There is still some risk of a few metal fragments left behind. Probing around with a magnet also helps on iron heads.

Many years after that, a neighbor trying to learn to do his own tuneups radically overtorqued the plugs on a late '60s 200 Fintail and stripped out everything, so I told him his options, and since the rings in the motor were pretty tired anyway, he decided I should risk doing the helicoil with the head on. I started looking around for a clothesline piece to try to stuff in the long 14mm hole in such a way that I could be confident of getting it out with some tweezers, when just then my wife came along and asked what I was looking for -- after my explanation, she said she had just what I needed, left, and came back with a couple of tampons. They worked perfectly.

But I think these newfangled back-tap tools are the clear answer -- in fact, I think that the genuine Helicoil kits have long had bits and taps designed to maximize the amount of shavings that are thrown out of the hole rather than allowed to drop in, but I also suspect that many times when people report no problems, it's actually because the shavings embedded themselves in the pistons without doing any damage to the rings or cylinder walls. I base this on the appearance of a piston or two in a few motors that I've torn down.