Always cap your bottle of fuel!!! It's not the nitro evaporating that's the problem, it's methanol's hunger for humidity. Humidity in the air quickly contaminates the fuel and your engine runs like crap.
Nitromethane has a pretty high boiling and flash point, much higher than methanol's boiling and flash points. You can extinguish kitchen matches in a pan of pure nitro without igniting it on a cool day. Yes, I have tried it, it's not something that I simply read.