Hello,
Should there be any conscious effort to build and maintain tension in the hamstrings and glutes at the start and decent of the squat? This would feel somewhat like a flexing the muscles.
Thanks.
Hello! I would not cue that at all. I’d want to see you get tight, overall, meaning brace, set your torso and take your breath. Then squat. 
Your hamstrings and glutes should then do their thing as you move.
I see…but does staying tight overall relate to the lower body as well or just bracing the torso?
Sure does, but you can cue yourself to brace and that should be enough. There are only so many things you can think about while moving under a barbell, very few that actually help. Trying to maintain tension that should be just happening as you lift is kind of unneeded mental focus.