If you are going to run RTF or buy exactly what the mfg recommends then you don't need a wattmeter. But if you start mixing motors, ESC, props and batteries, then a wattmeter is mandatory.
I have shown members of our club why they are burning up ESC, destroying batteries, etc. Usually they have too much prop or too little battery and they start burning things up. Sometimes they take a set-up intended for a 2S battery and they go to 3S. WOW, look at the jump in power! WOW, look at the cool smoke coming out of the plane.
Get a wattmeter and make sure you know what is going on.