I realize that the OP asked about US Plate Blocks but
it basically also applies to Canadian PB's I suppose.
I always preferred Upper corners and if I it was down
to just one then definitely UR.
No real logical reason except it just appealed
to me aesthetically.
Quote:
Matched plate blocks of all four different plate positions solves having to chose only one position.
I did that with many of the sixties Canadian engraved issues and then with
the imprint blocks, and housed them in nice expensive Lighthouse
plate block albums.

I sure wish now I had only bought
one UR.
And don't forget in those days for the 5c domestic rate stamps one had to buy
10 stamps from the Philatelic Bureau
to get
one Plate Block.
Twenty for the lower values but only the four for the high values.
The field stock sold in the Post Offices had the plate
inscriptions trimmed off.
That's why I really liked buying US stamps whenever we vacationed
down there since I only had to buy 4 to get a plate block
but usually ended getting six so I could have a couple of singles.