Is it safe to leave my Wi-Fi always on?

Yes you can and yes, it is completely safe. I never turn Wifi nor Bluetooth off on any of my iPhone's or iPad's.

Another point to consider is that if you want to automatically backup with iCloud every day/evening, you need to be connected to WiFi, have the device plugged in to charge and the screen locked.

It is a very good idea to leave WiFi turned on.