Also, the taps in helicoil sets tend to have straight flutes rather than spiral flutes, which will tend to allow the swarf to drop down the bore irrespective of whether the valves are closed.
Apparently the reason you make sure the valves are closed is to ensure any ally swarf doesn't get out down the exhaust port and bugger up the cat.
I've only ever done it by removing the head and it was a very straightforward job so personally I don't think it's worth the risk.
