Mitchum
Deity
Is it better to whip right after or right before a city grows to the next size? Granary obviously already long in place.
There isn't an easy answer to this question because "it depends..."
If you're whipping away citizens working unimproved tiles, shame on you... but also go ahead and whip away.
If you're whipping a military unit, worker or settler, more than likely it's better to whip sooner rather than later to get the most use out of said unit.
If you're whipping infrastructure, consider the gain from having that building one turn sooner vs. the productive tiles your soon-to-be-whipped citizens are working at the time.
The sooner you whip, the sooner the

Will whipping now cost 3 pops while if you wait one turn to grow it only costs 2 pops? Do you care about losing that extra pop? Are you trying to maximize whip overflow into something else where waiting a turn helps?
Like everything in Civ, it's situational. In general, whipping sooner is better in my opinion.