My understanding for turning on the monitor AFTER windows has booted up is to avoid any unfavorable signals....
Standard resolution arcade monitors need 15khz, but a standard PC vga card pumps out 60khz- which I've been told will ruin the monitor. Thus, people get software like Soft-15Khz and ArcadeOS to make their PC send out a 15Khz signal, but it will only kick in once Windows has booted up. The Bios and boot up screen are still at the normal 60Khz.
However, I'm no expert, and this has neither been confirmed or denied. I'm just sharing what I've learned from many hours of reading and searching.
Oh, and to answer your initial question, I just have the PC and monitor on separate circuits, so I just turn on the monitor once I hear windows is done booting up. Not as fancy as a "one button" start, but it works...