I've never pre registered hadn't heard of it on first cruise, but have never had less than a 30% discount. 30% on first cruise 30% on second , 35% on latest (but on dearer B deck not C). I Have pre registered early for 2025 in 2022
I think the 35% discount is a marketing illusion ( bit of a con). As almost no one pays the nominal full price, on my last cruise just before sailing still could get 15 or 20% on most cabins.
Nor should you pay full price, at the nominal full price Saga are not competitive, they as dear as luxury lines Practically pre registering helps you get the cabin you want , and a 5% discount, so definitely worth doing. Booking Saga at last minute except on guarantee basis is very bad value.