There are many factors involved with gaining maximum performance in an aircraft. Achieving the highest power to weight is necessary, but not the only consideration. For sport flying, your first consideration should be reliability.

While I don't find a tach necessary to set a 2C engine, I find that 4C engines are a different kettle of fish, one because you do not want to exceed rpm limits inherent with valve trains, and second because they are prone to back firing and throwing props if too lean. So use a tach to select a range of flying props and to richen up from peak rpm to avoid previous mentioned problems in the air.

2C engines are very flexible in operation. Changes in conditions (air pressure, temperature, and humidity) really effect the engine when you operate at the extremes, but have less effect for the normal sport engine. But understanding the basics of 2C engine tuning for the average sport flier can enhance the experience by eliminating unwanted dead-stick landings and poor throttle response.

So what tools does the 2C sport flier have at hand? Prop selection, fuel system, type of fuel, glow plug selection, and head shims go a long ways toward tailoring a power system that gives the performance you want. A tach is simply a tool to help evaluate changes, along with a critical eye during flight testing.