Hi Luis,
If one wire is shorter than the other you will have a slight offset in the stereo imaging of the amp, I don't think a few mm matters, but if you're dealing with more than a few cm, then this could affect overall imaging quality.......
Best to split the 2 rca inputs and put one of the sockets nearer the left hand side of the case, this way you can match the cable length for both channels, without having to 'coil' up the wire for one channel.........
Ofcourse if your not bothered than that's okay..............
Thanks
Raja