When I was using 2x drivetrain, I did feel the need for rear shifter markings. My drivetrain had difficulty shifting into the "big-big" combo and tried to avoid that. It did prove not easy to know what gear I'm in the cassette when riding up mountains with gently varying gradients without any reference how steep you're going. Sometimes you see the road looks flat but might actually be about 10% steep.
Perhaps, if my 2x drivetrain shifted smoothly on all combos, I won't need the markings.
1x is a completely different story however. The drivetrain shifted perfectly on all cogs. So I don't really care which gear I'm in. I shift when needed with total disregard on what gear I'm on.