Twice this year upon boating a King on Ontario and recheck my fluoro fly leader, I noticed my cinch knot had slipped to the point I was lucky I didn't loose the fish. The tag end of knot was pulled flush to the main knot. I think the stiffer nature of fluoro causes relaxation in the cinching over time where mono really cranks down on itself comparatively. Now these were knots that may have been tied two years ago so it might be good practice to either retie over time/usage or at least grab a pair of pliers, wet the knot and pull the tag end to retighten fluoro periodically.