I do it on an app-by-app, as-needed basis, if a certain app seems unresponsive for a long time.
iOS and iPadOS supposedly manage memory efficiently, so it's not clear whether it's really necessary to periodically close apps. However, the fact that Apple gives us the option to do so means that it's a fairly common practice, and often unavoidable. I can't tell you how many times I've had to force quit apps like Evernote, Dropbox and even Apple's Mail on iPadOS/iOS, just because they stopped responding for some unknown reason. I don't know exactly how it works when you've opened a whole bunch of apps in succession and iOS/iPadOS needs more memory to handle them, but I suspect that it's a LIFO (last-in, first-out) thing (hopefully a knowledgeable dev will correct me here if I'm wrong).