Short answer: I have run a single screw boat with a stern thruster and no bow thruster for 4 seasons. It did increase maneuverability, but it was a compromise compared to having a bow thruster. A stern thruster is far cheaper to install because you don't need to install a tunnel in the hull.
Long answer: My family has a Mainship habit, my parents bought a 1986 34' (single perkins) that came with a stern thruster installed. I was fortunate to have use of the boat and spend quite a bit if time cruising the Chesapeake Bay on it. The family had been lifelong boaters, but this was a significant step up from a 25' walkaround. My folks upgraded to a 390 Mainship (single yanmar) with bow thruster, no stern thruster. I've run this boat quite a bit as well, a bow thruster gives you control of the bow when stopped that you just cannot get any other way. Pulling out of a slip with a cross wind, there is no way you will turn a single screw into the wind without using a spring line. If you always pull into a slip bow first, a stern thruster would be more useful in departure, but when bow out, nothing helps departing a slip like a bow thruster for a single screw boat. Now the 3rd Mainship of the family is mine, a 1980 34' (single perkins) no thrusters. I am going into my 5 year with the boat and it gets a lot of use, often singlehanded. I am in a busy marina, pretty tight fairways, pretty well protected from the wind and not current. If I were to add a thruster, I would skip the stern thruster and go straight to a bow thruster. My boat does not respond to the helm at all in reverse, but a bump of forward allows me to nudge the stern in either direction. My slip is the first (or last) of the fairway and I frequently use a spring line to depart with a southern breeze.