Well, it depends on the design of the power supply and the computer itself, and unless you know the condition of the equipment, it's best to be careful, that's all. The likelihood that your equipment develops a ground fault is extremely minimal at best, and having an external "puck" or wall wart supply can possibly make the risk even less, since it keeps the AC mains outside of the computer. No need to have a paranoia about it!

However, there's a good reason why some equipment is provided with a grounded plug. If one is provided, it is almost always (depending on the design) connected to the chassis of the equipment (per UL/CSA safety standards) for your protection. Lift the ground pin, and now this chassis is electrically "floating." In physics class we learned that electricity always follows the path of least resistance. For the 0.01% chance you touch a ground-faulted (or "live") "floating" chassis while simultaneously touching something that resembles a "ground," you will indeed become the path of least resistance! This is why the ground lift adapter is banned in some countries!

I use ground lift adapters quite often in troubleshooting ground loops in professional AV systems. I lift AC grounds of various pieces of equipment (including computers) with the full awareness of what I'm doing, and exercise a good measure of caution. Once I have isolated the problem, I will restore all grounds, and then proceed to fix the trouble. I'm pretty sure I exercise great care when I do my work, especially if human livelihood is at stake, but you can never be absolutely 100% sure, which is why I have insurance!