A squared off rear will affect handling slightly, because as you lean over, instead of having a smooth curve from the center of the tire to the side of the tire, you are now going to have a sharp angle, so as the tire transitions from contacting the road at the center of the tire to the side of the tire, it will be more abrupt with a squared off tire.
A squared off tire is very noticeable on a motorcycle, but I have to be looking for it to notice it on a bicycle.
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Il faut de l'audace, encore de l'audace, toujours de l'audace
1980 3Rensho-- 1975 Raleigh Sprite 3spd
1990s Raleigh M20 MTB--2007 Windsor Hour (track)
1988 Ducati 750 F1