Site tubes are only needed if you with to quickly establish a system with your equipment. What I've noticed, is that I get to know my equipment, and I know exactly how much is in my keg.
Specially since my keg is marked into three different sections, and I know my keg is 15.5 gallons... so each "section" is about 5 gallons. I ended up measuring it and sure enough, 5 .125 gallons
I use floating thermometers, knowledge of my setup, and SS Spigots, and I'm quite happy.