its not about quality. Since you can export out at the same quality as prorez. Its about performance. Since fcpx has to combine multiple sources, audio, graphix, video etc, then combine them al into a whole, there is far more that could go wrong. Always export out a finished mastered rendered file and play that. Just make sure that the computer playing the file back can handle it. ie if you chose an uncompressed codec to export out as, the computer may not like playing it back. Choose a prorez or h264 codec, they will be fine.