I think it’s quite an interesting question, and I suspect the only real answer you could give is “it depends” which is the most frustrating thing to hear in the world ever, except for maybe “sorry, baby; it’s that time of the month.”
In my mind it depends on the purpose of the periodisation and how you want to focus it. If you were trying to peak for a bench PR or whatever, you’d start with the higher reps and work down. 10s, 5s, triples, doubles, singles, etc.
If your training was more physique focussed, your periodisation would be different. You’d be likely to increase volume, whether through sets per workout or increased training frequency. You wouldn’t necessarily be dropping into the lower rep ranges (although that’d work too).
So a 12 week, hypertrophy focussed (god, how pretentious does that sound) phase would be something like
-Weeks 1 to 4, 12 sets per bodypart
-Weeks 5-6, 14 sets
-Weeks 7-8, 16 sets
-Weeks 9-10, 18 sets
-Weeks 10-12, 20 sets
(EDIT: these numbers are totally arbitrary and I’m just using them to illustrate a point)
And you could get crazy with it and throw in intensity techniques like drop sets or whatever in the later weeks just to really ramp up the crazy.
After that time you’d probably be feeling like you’d had your ass good and kicked, so you’d drop back the volume by decreasing total sets, frequency, etc.
I suppose at that point you could switch to the lower rep ranges but I wouldn’t personally do that, as the whole point of the back off is to recover, not just brutalise yourself in a different way.
So… yeah. It depends. Periodisation is a state of mind, y’know?