I only read about half of the replies to your title, but IMO, asking 'To flush or not to flush?' is like asking 'What came first, the chicken or the egg?'.
There are always gonna be people stating the obvious.......... It does NOT rain enough to flush plants in the wild, but then there are those that will argue that synthetic nutes are not used in the wild.
My personal advice is get 4 plants, all the same strain, side by side, 2 with organic nutes, and 2 with synthetic.
Flush one of each when the time comes, and leave the other 2, and see which you like better.
Personally, I do NOT flush, I just taper off the nutes the last few weeks before harvest, and I've never had any complaints.
HOWEVER, I am one that makes sure I get plenty of runoff every time I water, not just a few drops trickling out.
For a couple Solo cups I might use a quart, for a 1 gal. pot, I would normally use a half gal.
2 gal. might get a 5 gal. pot and two 2 gal. pots watered, but usually it's a little more.