Nitrous pressure will increase as the bottle temp heats up, but if you keep the bottle covered it will be fine.
Im a big fan of automatic bottle heaters. If your running nitrous, its very important to keep your pressure around 950-1100. After a pass your bottle cools down, and your bottle pressure significantly drops. This will cause your motor to run excessively rich the next pass, since your bottle pressure will be lower, and your fuel pressure will remain correct.
They are also good for cool days.