The fast, smart-a$$ answer is "shorter is better".
Basically it's gonna depend on your exact configuration. Consider using a couple pieces of solid copper wire (Romex or HomeWire type stuff) as a mock-up to figure out the best routing, and get measurements from that.
As far as slack in cables goes - if you're wiring point-to-point between components that don't have any relative motion between them, and it's a short distance (~6"), cut the cable about an inch longer than the point-to-point distance, so the cable will form a gentle arc when it's connected.
If it's a
really short distance (~2"), a copper bus bar is better than trying to cut and terminate a cable that short.