I own a tach. I bought it a few years ago for an engine review I was doing just so I could show what the engine's RPM was.

Aside from that, I would never use one - especially for tuning a 4-stroke.

If used as stated, you should be ok, but as w8ye said, you will not over-rev it with a 13x6 - But here is where the novice can get into trouble with a tach...

Someone (or the manual) will say you should get (let's say) 9,500 RPM. If your prop (or fuel, or weather, or whatever) will only allow 9,000 RPM, but you keep leaning it out to try and reach 9,500, you can throw a prop, or damage the engine.

Just use a prop in the recommended range specified by your engine's manufacturer, and tune it like a 2-stroke. That is, start rich, lean it till it reaches it's peak (It will go slightly sick once you pass peak) then back it off 1 or 2 clicks from where it started to peak