The tach is a great idea, especially for 4strokes.

You will get advice that they're a waste of time, but truth is, they're an excellent learning tool. Anybody can hear rpms go up and down, but nobody has a clue what rpm they're hearing. And you'll never know from one engine to the next what they're actually turning. And no idea what the difference in rpm's is from one prop to the next. And no clue if an engine is running different if you swap it onto another airplane that resonates differently.

The tach will teach you stuff you'd never even think of if you didn't have it.

And after you've used it awhile, you won't find it necessary to use all the time. But you'll have it if you need it.