Hi!
Being a pylonracer at heart I use my tach for knowing how my engines react to different props/glowplugs/head shim/weather conditions and fuel. But not for setting the engines!
Even on the flight line when competing we don't mostly use a tach...We set our engines by ear!!! But if you cannot hear a 100rpm difference , go ahead and use a tach.

When flying twin engined planes I mostly use a tach for setting up my engines so I know that they turn approximately the same rpm (which isn't that important really). The most importat thing when flying a twin is to set the engines fairly rich so I can rely on them!

As I see it there is no difference in setting a fourstroke and two stroke...just listening to the sound.
But as guide line; Four strokes should be set so that they don't turn over 10000rpm.

So...For a newcomer; a tach is not needed at all!