Besides having the time alignment messed up with unequal lengths, you can throw the natural balance of the strength of the signal off. It may not manafest itself in lengths that are similar; but, the bigger the difference in length between the speaker cables, the bigger the difference in the loudness of each speaker will be if the balance is not adjusted. I have experienced this problem first-hand in wiring a multi-channel music system for an office complex.
I guess whether or not you find it acceptable depends on how hard-core you are about the hobby and what you are trying to accomplish.
Best of luck!