Position lights on in daytime make me think of that joke bumper sticker that reads, 'If you can read this you are too damn close!' If you can notice whether the position lights are on or off then you are very close indeed to another aircraft! Too close for it to matter much, I fear... but then there are people running around in catering trucks and suchlike, so I guess the lights 'on' option gets my vote.
In our outfit they are to be on per SOP, which relieves us of having to think for ourselves. Back when I used to be able to do that I think I left them off when in bright sunshine to save the bulbs but turned them on otherwise. Now I guess I would leave them on at all times as being a tiny fraction safer.