I don't like to stress about locking something out and whatever so I just simply don't desynth anything that has 100% desynth chance to avoid those +0.03/-0.01 moments.
So far at about 45, 42, 78 skill level for the 3 jobs I want to max out - desynthing stuff at 50%-80% chance gives from +0.29 to +0.48 or so with no or only 0.01 decrease. I don't buy items, just craft some if they require not many and easy to get materials or use stuff dropping from dungeons and leve chests.