Weight, height, where you ski(powder, crud, hardpack/ice), the particular ski, personal preference are all factors..
As i've found out trying a whole bunch of skis this winter, the particular ski makes a big difference too... some feel shorter than they really are, while others feel longer... I'm about the same weight as you...intermediate...ski mostly groomed hard pack, and have a used pair of 165cm Dynastar Contact 9s on order. The first time I went skiing, I was skiing 150cm rental skis and having a blast. 156 will probably be ok while you're a beginner...but once you get onto the blue/black diamond runs, a longer ski would probably help you...depends on what you're doing though.
So...it depends, if the ski will pay for itself very quickly, perhaps they're worth it... If you're new to skiing, and want a long-term ski, you'll probably want something a bit longer.