I tend to try and do top down. Prince rqts naturally do this, plus its easier doing them 2 piece as they usually have double grommets & hard weaves.
I also see a lot of rqts where the manufacturer has strung it bottom up.
To be honest tensions are lower and I sees squash rqts as structually sound as a tennis rqt, so bottom up should cause any problems. As for head shapes, I see no difference afterwards.
Babolat Strike 18 x 20; Wimbledon stringing team since '04 - Head Wimbledon stringer '14; Babolat stringing team '10-'14. Twitter - @therqtstringer