I have used the D800 without any grip for nearly 11 months. I did not have any problems when I upgraded from D70. It was definitely bigger and more solid. Actually I liked the more heft with the D800.
I just recently (a couple of days ago) added a 3rd party Mag.Alloy grip. I have not used it much to give any opinion whether the grip helps or not. But I am used to heavy body (used to F5) and I feel the extra weight helps in stabilizing the setup during hand held shooting.
I think the more you use D800, you will feel more comfortable as your hands will get used to the bigger size and extra weight.