[strike]I think that it's block-to-block for the NAV lights, regardless of engine state, but as always, don't quote me.
I've certainly never picked up a penalty from FSP for NAV lights, and I normally turn them on before engine start, off after shutdown.[/strike]
Well, half an hour skimming and searching through my "Big Boys and Girls Fun Book of Boeing 737's" (thanks again for that P-F) gives the following:
"Position (navigation) lights are to be ON any time the aircraft is powered."
In fact, the US airline whose AOM it is says the pilot should turn on the Pos lights prior to the external check (which for the 737 is, like, the first thing the FO is expected to do)
Can't get much more equivocal than that, I think.