On my server it really does vary but generally most days between 6-8 GMT Republic will seem to win more than 50% of Warzones. Other times such as last night between 8-10 Imps were winning everything. There were simply not enough experienced Reps to compete. This shows the time you play will have a big effect.
I really think two other things are happening here. One, each server is different and you can get unlucky or lucky in your faction allies and/or faction enemies. Two, most important, is this MYTH that seems to have appeared that the Imperials win so much more in PVP. It's like so many Republic players have lost the battle before its even begun.
Edit: Is this Myth based on Republic players playing the "good guys" getting more raged when they don't win (as the good guys nearly always win right?) and the Imps can take a defeat better? I just don't know. Also, I play Republic.