For my megatree, I put 6' cords on the LEs. It makes for a bit of an octopus to untangle, but I didn't need any additional cords for the megatree so it is a reasonable trade-off. For other parts of the yard I made legths depending on where the power was needed. Using SSR4s for the Icicle lights, I did 15', 6', 6', 15' for the four outputs. Again, I only neede an additional 3 foot extension for one end... For the star on my megatree, the SSR4 is mounted on the star and has only one outlet with a 15' cord. The other three circuits are wired directly into the SSR4 with no plugs...
Needless to say, I had the whole layout planned before I started buying and putting stuff together. I figure each year I will be adding to the previous year, and if I need to change the lengths, it's just a couple of screws and a zip tie per cord...
RM