I just went through this myself, bought some bulk DH Labs Q10 Signature wire. I need 15' on the right speaker, 7' on the left. It's cheaper (the wire is $15/foot) and easier to cut the wire to the lengths required, however come resale time you're kinda behind the eight ball. There's no harm in using equal lengths, so I bought 30' of wire and made them equal. The extra 8' of wire I don't need for the left speaker is tucked behind my audio rack.
Compensating for different speaker cable lengths
The minimum speaker cable run is 16' to the right channel of my system, but only 8' to the left channel. If I add a second 16' cable in parallel to the right channel, this will equalize the total resistance of each channel (although inductance and capacitance would be different). Alternatively, I could simply use a single 16' cable for each channel, or purchase a 16'/8' pair claimed to be insensitive to the differences in length (e.g., Ocos or Transparant). What is the best general solution to this problem relative to soundstage imaging and frequency response?