Should I disassemble and then reassemble something that works?

I am trying to get into repair/ improve my repairing skills primarily of the various damaged electronics we have lying around but also noticed on the site there are teardown guides and I'm wondering if that is a good way to expand my knowledge/ skills is to teardown phones and other things like lawnmowers for example even if they work. Is that a good idea? I'm afraid if I do teardown something that works I won't be able to get it going again and not have a clue why it's not working. Thoughts?