I grow in pots of Hydroton/growrocks in a flood and drain tray. Been working with this system for 9 years now.
Some guys/companies would say yes you have to blast the plant with P,K in bloom. Many of them heavily use or sell lots of additives and boosters...........
First let me say I'm not a botanist, and I don't mix my own nutes or anything. I'm not gonna claim to have all the info, I'm just sharing my understanding of things I've learned over the years. I'm just a guy that has tried some stuff and has done a lot of reading.
From my reading and experience its more about the ratios, concentrations and accessibility. The right ratios of NPK for happy growth so that the nutrients work together properly and in balance providing accessibility of each nutrient to the plant. Then feeding at the correct concentration for the stage of growth you're in to keep a healthy plant.
Most of the "all in one" nutes have plenty of PK for flower and they just get used in lower doses during veg. They have a proper ratio, you just have to change the concentration depending on how much your plants eat.
Then you'll ask well isn't that to much N for flower...right? The simple answer is no not usually. Green plants are healthy plants. Healthy plants produce the highest yields and the most cannabinoids. I try to keep my plants nice and healthy especially while they are flowering and working so hard.