When you are doing 9/10s of a wheelie already, stomping on the gas is a bad idea.
Even so, most folks into this recreation get a little tipsy occasionally, more so than just the normal 1 tire in the air variety. First time I got to the tipsy point was inching across a very deep rut at an angle and the truck popped into too much of a lean with one rear tire in the air and one front tire barely on the ground. Several guys were right by me and quickly through their weight on the back bumper to keep me grounded enough. Another time, I was taking a bypass around a pile of rocks and threading my way between trees. I have no idea what caused it but for a second, the truck just popped up and almost went over. After getting through this, I inspected things to see what I missed and neither me nor any of the other folks could figger out what had caused the unstable pop up.
One of the other posters mentioned insurance excluding off road mishaps. This exclusion is not common at all. Your insurance company may cancel you after the fact but that is not much more likely than getting canceled after any other vehicle accident.