Re: If you need to be seen, sidelights.
Personally I take flashing headlights as an indication that the driver is awake and watching the road (as opposed to operating their mobile phone or shaving their bikini line or trying to slap a child in the back seat) so I'm slightly more willing to turn across or pull out in front of someone who has just flashed their headlights than someone who hasn't.
On modern cars there is no option to turn on sidelights without (dimmed) headlights.
Unnecessary full-on headlights annoy me because of the dazzle. On an overcast evening, getting towards dusk, but before the street lights have come on, it's harder to see the cyclist in front of me if there's a twat with superbright headlights coming in the other direction.
Biting the hand that feeds IT
