Trust the guy who learned the hard way with his own warped deck. Wood moves more than people realize, especially under that hot sun. A quarter inch gap now prevents a cracked board later.
People never believe how much wood can shift until they see it happen. I watched my own deck planks push each other out of line over one hot season. That quarter inch gap sounds tiny, but it's the difference between a nice deck and a DIY disaster.
It depends a lot on the wood type and your local weather. Pressure-treated pine can shrink a lot as it dries, so starting tight can leave big gaps later. Hardwoods or composite boards move differently. Your neighbor might have gotten lucky with a dry batch of wood that already shrunk before install.
Seriously? I built my deck tight five years ago and it's still flat as a table. Does every single board really need that much room to breathe? My neighbor did the same thing and his looks fine too. Maybe people are overthinking a few cracks?