If you're about to break in an OS, and have read and understood their recommended break in proceedure, no you don't need a tach.

And if you don't understand it, but can figure stuff out while following orders, no you still don't need a tach.

On the other hand, if you're about to break in a 4cycle, yes you need a tach. And will still need to figure out the break in instructions.

Still awake?

Beginners need to learn what "rich" means. "Lean" and "two cycle" isn't hard to hear. Once you get used to identifying when an engine goes into lean/rich, you pretty much don't need a tach. Unless you're planning to fly pylon or such.

But back to the original question but let's phrase it differently. Will a tach help you break in an engine? No, for a simple reason. Nobody publishes the rpm your engine will be turning when it is rich or the rpm when it breaks into a two cycle, or any of the other numbers.

Unfortunately, a tach won't help most experienced modelers break in any engines since the engine mfg's don't tell you numbers. They can't. The charts and graphs would fill a book bigger than the engine boxes. You're going to have to learn to identify "rich" and "2 cycle" by ear.