Wire length from the battery to the ESC matters a lot.

Wire length from the ESC to the motor matters little.

Castle has guidelines on their site somewhere, but I couldn't point you to just where.

Keep it as short as possible, too long motor to ESC can in some cases cause erratic commutation and too long esc to battery can cause overvoltage - both can damage transistors. The rule is, that if you have battery voltage close to max voltage of ESC, you cannot lengthen battery leads, and in that case you must do it on motor wires. If you have low battery voltage, it is better on battery side. However proper way is to lengthen battery lead and add capacitors. They should be installed in case that wire length is over 10 inches. But it depends on battery volatage (difference between battery voltage and max ESC voltage)

True, but one of the advantages of an electric twin is that you can shove the battery all the way forward in the nose for balance.  If you put the batteries in the nacelles then you're limiting yourself to how short they can be.

Hi Paul,
I see that you've gathered a bunch of good info here.
"Doesn't matter" is a bit of an overstatement, but yes the ESC is much less sensitive to long motor wires, provided the three wires are twisted together neatly and tightly. This reduces the risk of broadcasting into the cotrol cable which should always be run far from both sets of high-current wiring in the airplane.

I'd hate to put batteries in each nacelle: the bar-bell effect of two batteries out there will cause tip banging and hinging in the crossover transitions that no amount of trimming may fix.
Fortunately a pair of small light motors won't cause too much problem that tip weight and flap asymmetry won't do the trick.

take care,
  Dean P.