You don't have to carbonate it while it sits at room temp, but you can. You'll need to use a carbonation chart and set the pressure according to ambient temp. The thing is though, you'll have to set it close to 30 psi to carb at room temp. If you have keg(s) in the frige already that you are serving, you won't be able to set the pressure that high unless you have a multigauge regulator because that pressure is way too high for chilled beer. What I do is just purge the air out and then give the keg a quick shot at 20-30 psi. This seals the keg up good and it willl stay sealed until you're ready to chill and carbonate.