we normally use c9 strings spaced 12" apart, but we also have used 15" on some taller residential homes. we have used 18" to 24" spacing on 3-8 story commercial buildings, and 36" spacing on stake lights for edging drives and sidewalks.
I had one year I used a lot of 15" on homes, and it does save electricity and bulbs, and looks ok unless viewed side by side to a 12" spacing house. I believe Christmas Decor uses a special 14" spacing, which is fairly unique to them, and not really available to us. it looks similar to 12 or 15", but it lets them save a few bulbs. With led bulbs, saving 25-100 per house adds up.
I still only use LED bulbs when asked by customers to do so. I have several projects that have them, but only the newest 7 and 9 diode and best color bulbs look good to me, and even then they still are directional. they always look brighter viewed from the top of the bulb rather than the side. The ones I usd 3 years ago look pretty dim and cheesy now. and they burn out like crazy. "50,000 hours? " .while I can replace incan bulbs all day long, replacing LED bulbs hits your pocketbook pretty fast.