Hendrix was spontaneous. He'd just go out and get in the zone. He Didn't tape spots or sound check all that much... He tried to NOT feedback. And You don't have to find a spot to get feedback when you have headroom enough to crank up the level to feedback volume. He could just turn up the volume-step on a pedal, etc. He tried to keep it down and under control. Stand in front of three Marshall 100 w Plexis with single coil, it's not an issue trying to get feedback. BUT > I know of people that did tape spots, like Gary Moore. He taped the floor at sound checks — BUT the difference is, he had certain numbers where he played at low levels soft and slow at first only to bring the song to a climax on a certain specific note that had to sustain to do the song justice.
And he'd be playing to an audience that wasn't there particularly for real loud Festival rock levels. So, like in smaller venues, he had to make advance arrangements. He had to have things organized for feedback to hold, e.g., one note in that one song, at the exact time, with no other collateral notes. So he would find the spot at sound check and, yes, he would tape the floor.