I would sum things up this way, if your machine can handle the codecs of your media with no issues, then dont transcode.
If your computer has problems dropping frames or other issues, , first determine the cause. The Mercury Playback Engine thrives on power for the CPU / Ram amount / CUDA from the GPU and I/O speed of media accessed off your raid / hard drive.
Some systems might not do as well as others if some of these are old or lacking. Transcoding might be a consideration in that case.
Tom Daigon
PrP / After Effects Editor
http://www.hdshotsandcuts.com
https://www.youtube.com/watch?v=DxPrG3WUyz8
(Best viewed at 1080P and full screen)
HP Z820 Dual 2687
64GB ram
Dulce DQg2 16TB raid