Is grounding important? Is the grounding wire forwarded to the Magsafe connector? There are these 2/3 pin adapters and it's so much easier to convert a 2 prong connector while travelling than the bulky three prong ones. I do know the dangers of not grounding an equipment that needs grounding but -- does the Macbook Pro need it?
The short answer is no.

If you remove the cable or "duck head" from your AC adapter, you'll see that there are only two prongs that actually make contact with the power cable. The ground wire doesn't make it to the adapter. You're safe to convert the cable to two prongs.