A tachometer has its uses. I keep one in my flight box, and use it to set the high speed needle on the first flight of the day. When I set the idle, sometimes I'll just check to see where it's idling (2500-3000 is usually a good target, but it varies).

For most trainers, a very low reliable idle makes landings easier. As you move up into planes with heavier wing loading, you'll find its not so critical, since the planes tend to land with a little more speed (again, this is plane dependent). In those cases, I'll set my idle a little higher, as it makes the overall tuning a little easier (I'm talking a couple hundred RPM, as in 3000-3500 at most).

But Ken is right. Set it (once a day for me) and forget it. As for the low end mixture, once my engines are broken in, I've never had to make any low end adjustments after I've got it set where I want it. If the low end is getting saggy, check your glowplug.

Brad