Well yes and no IMHO. I mean, it needs to strenuous but I don't think it has to be a true heavy single. Case in point, you can learn a lot of things from watching someone do a hard set of 8, because typically the things that give out first as fatigue sets in are the weak ones. Any heavy triple would probably do too...I'm of the opinion that unless you're really really efficient at the squat, most beginners/intermediates form will vary from rep to rep, some being better than others. So multiple reps help to get a sense of the "average" form, for lack of a better word.
Also if you're unused to heavy singles then 90% 1 RM could be just ugly anyways