Sushi in lunch box? Sushi rice preservation
If your primary motivation is food safety, then you shouldn't keep rice at room temperature for extended periods of time.

There are some people who say that leftover rice shouldn't be kept at all, because there's a risk of Bacillus cereus multiplying between the time that you cooked the rice and got it chilled down.

As such, you should cook your rice, cool it down to make the sushi, and then either consume it, or refrigerate it immediately.

Also possibly of interest (although I don't know if it would work for sushi), scientists have found that adding coconut oil when cooking rice, then chilling it can convert sugars to a non-available form, if you're concerned about diabetes risk.