That's a good point. As long as the head tube is faced correctly you can't really mess it up unless you rush. The cup will self-align to the head tube so just don't damage the cup or head tube and it'll work out for you.

My coffee mug method or the block of wood and hammer method just requires that you tap carefully and ensure the cups are sliding in straight. If not then adjust it with some taps on the appropriate high spot.

It's not a nuclear reactor...

FWIW, on cheap bikes we'd install the cups with a bench vise and a sacrificial strip of wood or plastic.