I try to never allow old fishing line back in the water. I have heard tales of fishermen actually drowning because they became tangled in discardrd fishing line. I also cut up 6-pack holders to avoid injuring fish or birds if they get back in the environment.
I am very careful with monofilament.
I had to cut off 10 ft of tangled line last night and it was wrapped up into a nice little ball and safely stowed in the tackle box before I went back to not catching anything.