Standing up I have found gives your voice that extra emphasis just when you need it. It gives you a maximum range of options, like bending your knees or talking with your hands. You will find that your voice will have an extended range by being able to take deep breaths because you have proper posture. Try standing the next time you are voice acting a bold energetic piece so you can move a bit, and swing your arms, or whatever feels natural to you.
Let's face it, sitting is more comfortable but compromises some of the bigger movements. Standing for too long is also impractical and can affect your voice after a while because you get tired. I don't think I am going out on a limb here; sitting is not a bad thing when recording an audio book for 10 hours. I know a lot of people record sitting down especially during long narations. Setting can stress your voice if you don’t get the proper air flow due to improper posture, so to improve your sound quality set up straight and shoulders back. Oh and make sure to oil that chair the last thing you want to hear in your recording is your squeaky chair.
So I think in the end it’s up to you and your voice style.