I always find when I do the uni-knot as a loop (davy loop), just the act of casting and retrieving tends to tighten the knot, but I've never tried to loosen it. I've been practicing the mono-loop recently instead. I do like the uni-knot for attaching to the hook bend. I haven't done it enough to have any come off yet, but I usually do a minimum of 3 turns. Usually I do one less than the tippet size i.e. 6x gets 5 turns.