I'm still in the testing phase of Premiere DC, to know him better.
I noticed a deterioration in quality just amount mpg files, they are old movies on VHS, and I do not know what to attribute this fact.
With the old release CS5.5 this problem does not exist, these are the screen (previews of the timeline):
Premiere Pro CC
Until the problem is resolved, I prefer to work with the old release CS5.5, I do not feel to risk with the new version.
I've seen this with some XDCAM clips too, which are also MPEG based. It's a bug with Premiere Pro CC, I believe. You should report it to Adobe.