When growing AFs in soil, I start feeding fish emulsion + liquid kelp at week 2, gradually increasing nute concentration to reach full strength by week 4 more or less (ie. week2 1/4, week3 1/2, week4+ full strength) until week 6 is done (about 3 weeks into flower). Then I stop giving fish emulsion, feed liquid kelp + molasses (1tsp/gal) during week 7, and flush until the end (at least 2 weeks, if not 3). I often use an enzyme product during feedings.
In other words, I do feed N until early flower, and have found no negative effects by doing so. I like them as vigorous (ie. big, branchy) as possible before they start budding so I may be pushing them a bit but I'm quite satisfied with the results.