I was just wondering... since starting a new save without quitting first causes corruption and all. Is quitting first important at all times or only when creating NEW saves? Is it best to also do it when I'm loading up another save?

Well I’ve heard sometimes it causes the ghosts to be treated as households

I’m wondering if it’s okay to do it without quitting when loading up an existing save though.

I don't know about corruption. What however happens is memory problems. Your next save will not have enough memory available because the previous one will have not discarded all if you don't close the game completely. That happens regardless of whether you load something new or not.

Also I seem to recall that, if you have certain cheats active in a save and forget to de-activate them, they will still be active in the next thing you load, whether it's an existing save or a new one. Things take longer to load in either case. Best practice is to always quit the game first. Making sure you have saved any progress you want to keep, of course!

The game does not "tidy up" well enough just going back to the main menu and starting a different save, no matter if it's an existing one or a new one. Some data will "bleed" into the next save. This for example affects lighting or cheats, sometimes sims attributes or other things.

So it's better to quit first before starting a different save.

This is actually intentional, not a bug. EA failed to have TS3 clear the contents of RAM in between game save loads (new saves or existing, doesn't matter) strictly to mess with our heads. It's all part of a complex psychological experiment and they have been monitoring our reactions to the messes this causes ever since 2009. We hope the results of this experiment will soon be published in the appropriate technical journals.

Okay, it's a bug. Our recommendation is to never load up or create a save from the Main Menu after having offloaded one without quitting and restarting the game first, no matter how inconvenient that seems.

Existing save is bad too. I found this out the hard way. The cemeteries empty and the houses fill up with wonky ancestors, which I suppose could be entertaining, but the save is corrupted so we'll never know.