You do not want to run two different lengths of wire. See the recent "Cable lengths" in cables for the reasons why.
Keep your speaker cables the same length. I have been faced with the same dilema, so what I have done is to coil the excess cable behind my rack. One option that I have been considering for a while is to purchase an AV table/rack to place in front of my RPTV. This would give me center massed storage and a stand for my center channel. If height is an issue for this solution you could have a solid riser built for your RPTV with a tunnel through the center to run cables through.
Just some thoughts.
I recently found a good solution for me to this problem. My wall for the main speakers and a large screen was just too tight with the equipment stand on that wall as well. I moved my rack to the back of the room, & used 25 ft pre-amp to power amp runs for the audio, purchased a high quality 7 Meter s-video cable (around $160). Now the 2 speaker sound is much more open, the front of the room does not look so cramped and the preamp controls, DVD player, & CD player are not much more than an arms length away. I received a confirmation from Audio Quest that preamp to amp runs can be even much longer than that without an impact to the audio. I did not have any apparent loss of audio signal and in fact the equipment spacing improvement helped a lot!