Before getting the iPhone X I used to fill all 5 of my fingerprint slots for Touch ID. I'd register the thumb and forefinger on each hand and then re-register my right thumb (probably the most commonly used) but starting with it in a different position.
I found this second registration gave me maximum reliability regardless of how I was holding the phone.
My one complaint about Face ID over Touch ID is the relatively narrow range over which the sensor will recognise you. I've reset it and re-registered from a lower angle but it's still not perfect.
The news that iOS12 will allow for a second face to be registered (as long as the feature makes it through the beta process) would let me replicate this behaviour with Face ID - to a degree.
Re-registering a second instance from a different angle will make Face ID more reliable.