@lukasbodo,
Without getting too technical this is what I know.
If the leaves are no longer green, then they can no longer photosynthesise, thus they are no use to the plant.
Alternative to this being that if they are dry but still green, then a bit of watering and they should be able to work again.
To answer your question when my leaves have yellowed I either trim down to the green parts, which can still photosynthesise or I completely chop off.
Hope this helps