A corollary FWIW.
I recently encountered an issue when I ordered some replacement LED bulbs for my fixtures. It turned out the particular bulbs I got are polarity specific; they only worked in the fixtures where the positive was wired to the bottom of the fixture. They are standard e26 bases (screw in type) that I got from a company called Wattalight.com.
The company is very receptive to returns, but in this case I think I will just reverse the wiring
since they are in Canada
and it might cost half again as much for me to return them. The lights look good and they were about half price
of Dr LED or Luna Sea, although I don't believe either of the latter two are polarity specific.
There may be more lights out there that are polarity specific (and not stated so on the website - the drawback to Wattalight), but if you are wiring
the fixtures from scratch, it may be best to try to wire them all up with the positives in the middle location, but I have no idea if other lights might have a specificity in the opposite direction.
Just my 2 cents