Can't really say if taping one or two holes would be right - only you could answer that, but if you closed offf sufficient holes such that the motor was getting the same amount of air as it was when the air was warmer, you'd be safe.

I'll assume you've heard of stochiometric ratio?
http://en.wikipedia.org/wiki/Air%E2%80%93fuel_ratio

Ultimately, that's what you're trying to achieve...

And Bully, of course you should have an NSR! If you jet on the safe side it'll be absolutely fine whatever the weather, it will simply run a little rich in the summer and not at it's best.