It is not safe, even if you have a stable electricity current supplying your house.
If you live in an area where the electricity lines are unstable or inconsistent, I wouldn't risk it unless you want to fry your surge protector, as the third prong is what is used to ground the plug, meaning that any electrical surge means frying electronics! 🙁
If you feel confident that you're willing to risk stuffing up your power supply, you can by all means use a cheater plug just fine in normal use, but any spikes in current from the wall socket and your PSU is going to be dead.
Pretty much don't do it full stop though, not safe.