How many miles per day are you doing? The less ambitious your mileage, the easier it will be to target a specific campsite. And/or: be willing to hike 6 miles one day, and 14 miles the next.
I tended to like the high-altitude lakes. Assuming a good weather forecast, next time I might attempt to time my camps for: Lyell Glacier (a little off-trail just before Donahue Pass); one of the gorgeous lakes atop Silver Pass; Selden Pass (ideally Marie Lakes); Evolution Basin (any lake between Evolution and Wanda Lakes); tarns in Upper Basin (south side of Mather Pass, right as it flattens out after the steep descent); anywhere in Rae Lakes Basin (Dollar Lake or above); the tarns above Guitar Lake (for an early Whitney start).
Of those, an Evolution Basin campsite would be my top "Can't Miss."
For some reason I have not had good luck locating a campsite that really impressed me anywhere between Forester Pass and Guitar Lake, at least not strictly along the JMT. Maybe someone here can fill you in. Wallace Lake (a couple of miles on a use trail to the east of the JMT) is a stunning location, and I might try that next time.
Great camping with good water access can be had for just about the entire length of the trail. So an argument can be made to not overthink it, and just camp wherever you find yourself at end of day.