Do I need to keep iCloud back up on constantly

It's certainly the normal thing to do. That way, if you ever need to restore from a backup, it's as up to date as possible.

iCloud backups occur each day when your device is connected to a power source, connected to Wi-Fi, and the screen is locked. For many people this means overnight when their device is on charge.

For more information, see How to back up your iPhone, iPad and iPod touch.

You get 5 GB for free, while the cheapest subscription gives you 50 GB. That's more than enough for iCloud backups, and you can always increase it later if you need to. Here's how to buy more iCloud storage.