It's been a while since I've tried to turn on and off mods like I do in other games *cough*Skyrim*cough*

Let's say I were to remove a mod from my list and reload a world that had that mod. I recall from back when villages and villagers were first introduced that my friend's private server had an issue with generating handheld maps; they would crash the server on our world which used to run tekkit, but had to be removed due to complications.

Tangent aside, does that still happen?
Or does removing a mod from the folder and then reloading the world just change every mod block that exists in the world to air?

Is is still just safer to load an editor like MCEdit and manually remove all mods blocks first before uninstalling the mod itself?

Answers appreciated, thanks for your time!