It’s important to add the vinegar mixture to the rice as soon as it's finished cooking. If the rice sits too long, it will be difficult to season evenly. The bites should be assembled just before serving, but you can make the rice up to 1 day in advance. You can chop the tuna ahead of time and keep it chilled, but don't toss it in the sauce until the last minute or the acid in the sauce will start to change the texture. Depending on your altitude or climate, you may want to adjust the amount of water or rice when cooking sushi rice. If it's really dry, increase the water a little bit (or decrease if humid). If high altitude typically affects your cooking, add a little more rice (about 1/4 cup).