I have a lot of cross country flight time in 3 different RV-12's (some of it on trips of 1800+ miles).
I use the site window to verify fuel levels in comparison to the electronic fuel gauge and fuel computer, before and after a flight.
After some experience, it is easy to guess within a gallon or so, what the fuel level in the tank is based on what the level is in the site window.
I have never splashed fuel on me when refueling.
It is easy to recognize by sound when it is getting nearly full without having a gauge to look at. The only time it has been challenging is if a fuel truck is parked very close and is overly loud (only happened once or twice).
I am not meaning to put down anyone that chooses to install a mechanical gauge, but I do think it is a higher level of risk to rely on a mechanical device to confirm a fuel level before flight. (And I do kind of wonder when someone blocks off the site window, installs a mechanical gauge, and then uses a dip stick to verfy the fuel level...... kind of seems like a step in the wrong direction

)
It has been a long time standard to use your eyeball and visually verify a fuel level before flight.
That is the entire engineering basis for the site window... walk up to the left side of the airplane... jiggle the wing while you look at the window... verify that the fuel level is above the 4 gal. minimum fuel for take-off limit... go fly!