I use the same nutes I use in veg. I just add MKP at flower. The plant can benefit from additional P/K at flower just like any other flowering plant. I use VitaGrow. Three parts. Calcium nitrate and micronutriens during veg. At flower add the MKP on top of what you were feeding in veg. The plants can benefit from more p/k at flower but there should be ample amounts in any decent base nutrient. When I say decent I don't mean fancy labels. I mean a product that delivers what a plant needs. What I feed does that. I'm basically using the same chemical salts in dry form that many nutrient manufacturers water down and put into multiple bottles. I will admit that I have added small amounts additional potassium sulfate and/or magnesium sulfate during flower but I didn't my last grow and didn't notice any difference. I just see too many people in the Marijuana Plan Problems section using ten or more bottles of stuff having nothing but problems. I never have any issues. My plants grow healthy from start to finish.
As for what
@homebrewer is saying, I agree to a point. It depends on what's in the veg nutrients. Some do provide all the P/K a plant needs. The Vitagrow I use is three parts. Calcium Nitrate 13-0-0. The calcium nitrate is also 13% calcium so you'll never need any CalMag. The micronutrient is 3-10-20. It has all the magnesium, sulfur, iron, copper, etc... plus ample amounts of P/K. Those two parts are the veg nutes at 16-10-20. MKP is added at bloom which is 0-49-29. I could probably just use the two parts without the MKP and not notice any difference in yield or quality. Some veg nutes have everything in them. Some of these nutrient lines have everything split out into different parts and feed charts that have people feeding a dozen different things at different amounts every week. It's a joke. Break out the eyedroppers! I'm feeding my plants!