I try to relax my upper body and not pull or push on the bars. Sometimes it is inevitable and I do pull on the bars, but I generally focus on keeping up a quick cadence (for me) and keeping my pedal strokes even. Greg Lemond talks about this in his bicycling book. He talks about the differences between putting your hands on the bar tops versus the brake hoods.
Bottom line, there is not right or wrong way. What is comfortable and efficient for you?