Chill dude, no response in 90 minutes doesn't mean no one knows the answer. Ideally yes you flush with pH-buffered water. Remember that the pH you are most concerned about is your runoff, which could change after the flush because your runoff pH is the result of the interaction of your water, nutes, buffers, and your soil mix. If you are talking about a final flush, a week or two before harvest, then keeping your pH in line is not as crucial because the main reason you are concerned about pH is the plant's ability to uptake nutrients, and after the final flush you stop feeding anyway. If you are flushing at any other time of the grow then you should use pH-buffered water.