Tachs are great but I was flying for a couple of years before buying one. I generally use it if I am changing props, etc to see what kind of difference I am getting, like what darock. But most of the time I tune an engine by ear. Theres no doubt when an engine is running like it should. It takes some time to get a feel for it and is more obvious on 2 stroke than 4 stroke engines.

Do I like my tach? You bet. Do you have to have one? No, if I was a newb there is probably a lot of other stuff I would try to spend my money on first (starter, power panel, building tools, voltmeter, etc) But thats up to you. When your starting in the hobby it takes a little while to get all the bells and whistles.