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I thought that especially in the case of yellowing, the healthier growth is actually leeching sugars and nutrients from the lower plant to sustain itself.
If that's true then when you remove a leaf you'd always be throwing away some goodies. Maybe most of the time it's just not that critical...
Personally I've been doing the 'if it lets go of it with slight pressure' approach.
I guess ultimately it comes down to aesthetics... In nature it would take an another animal brushing against a plant or strong weather to remove sub-par growth.
I'd be willing to say a leaf is never hurting its own plant aside from infection or infestation, which are both parasitic circumstances.