Some growers don't flush at all. Some growers flush 14 days prior to harvest, some 7 days prior to harvest, some 5 days prior to harvest. There is so much info out there and not a whole lot of science. So it's your choice really. Here's a video I made about a study done on flushing:
I always ensure the medium is in the proper pH range for optimal nutrient uptake. So if you need to pH your water, go for it.
@Mr. Grow It a lot of great info in that video 👍