No reason to keep either on since they drain the battery even when not actively browsing the net. And if your LTE can’t find a tower to connect, it keeps trying to connect, draining the battery significantly. This was pretty bad for me a couple years ago; not so much anymore since I live in a city now but I still keep both off, mostly habit but also because I like having my battery above 90% while it’s sitting idle.
When I’m at home and want to use the net on my phone, I turn WiFi on, then off once done. When I’m outside the home and need the internet, I turn LTE on and then turn it back off once I’m done.
My only issue with RCS is that it requires that either LTE or wifi be on. I personally keep those off unless I want to actively use the internet. Not a big deal, minor annoyance though.