I run 15% Omega in all my engines, 2- and 4-stroke, for the same reason Minn said, it's just easier that way. (while I've heard that caster in 4-strokes can gum things up, I've never had that problem, so I keep doing it).

As for a tach, I have one, but rarely use it to set my engine mixture, espeically not on 2-strokes, and usually even not on the 4-strokes. I do either a pinch test or a lift test and listen for RPM changes. Even on the 4-strokes, you can hear the rpm change. It's not as obvious, but it's there. Part of the reason I don't use it is that unless you get a really good tach, the readings can vary a little, and some tachs don't settle down to a "rock solid" number very quickly. So you spend a lot of time waiting for the tach to settle on a number between each adjustment. I find it faster and easier to just use my ears. (of course, if you are the type that has a hard time waiting for the engine to react to a needle change (since it's not instant, esp on remote needles), the tach might help you remember to make your changes slowly and let things catch up to your changes. YMMV).

Also, most 4-strokes have very definate ways of telling you when they are too lean. Usually you get a knocking/pinging sound first, then you get the prop leaving the airplane . (no, I don't set my 4strokes by listening for the knocking and backing up from there, but I know that if I hear any kind of knocking to throttle back and fix it. Espeically when you have an engine that is going lean in the air.)

(mostly I use the tach to determine how well my engine is running, which engine is running stronger, and evaluate props, plugs, etc. Since I fly combat competitions, it's a useful tool. Espeically since one class of combat has an RPM limit in the rules)