Would not hurt, especially if the spinach was a bit wilted.
If we really want the nutrition of spinach or kale, we just need to juice a lot of it and put up with the waste. It does yield some juice, just not very much.
If you use whole kale leaves, the big ones, you get a bit of juice but most of it comes from the stems because they are thick and fleshy and juice good. The thin leaf portion just doesn't give much. Which is why you should not use bagged kale, that is mostly leaf and not a lot of the thicker stems and will therefore not yield much juice..
What I do when I really want kale juice is I put the juicer on high speed and very very slowly feed the kale into the juicer one leaf at a time. The key is to do it slowly so the juicer does not slow down. That gives me a bit more juice than just balling it up and shoving it through the machine. Never do it on slow speed - you want the basket spinning as fast as possible.