Sean, I'm aware of two claimed disadvantages of unequal length speaker wires. The first, more commonly raised, that there will arise a "timing" difference between the speakers, is total nonsense. Speaker wires carry the signal at speeds approaching that of light(close to 186,000 miles per second). The differences in length that could possibly exist in the home result in far less than a millionth of a second delay and are meaningless.
The second complaint is that the greater loss due to more resistance in the longer wire will "unbalance" the sound. This would be theoretically possible, but doesn't happen with the gauges and lengths of run used in the home. As the handy table at the end of
this article shows, even the difference in loss on your 10foot/40foot combination would only be about 0.1 dB(out of maybe a 70-80dB average sound level). Moving your head a couple inches or putting a pillow on one side could have more effect.
Of course, as Bill and others have mentioned, for a totally different reason very thin wire shouldn't be used on long runs(e.g. 24ga on your 40 foot run).The table
here shows recommended usages to avoid having a too-high resistance interract with the swings in speaker impedance and cause fluctuations in frequency response.
So, the differences in length which you propose would cause no problem, and in addition the 12ga wire would be more than enough for even your 40 foot run.
