should I be closing out apps on I-phone 11?

Unless the app is misbehaving, there is no technical reason for closing it out. Some people do it because the find it easier to switch between frequently used apps if there are few of them open.

I find it a waste of time but it's a matter of personal preference.

Force closing an app is not required. If the app is used frequently then opening the app again and again may consume battery power each time it opens up.

It is my experience which says the app which is frequently used has to be left open.