I talk about top screw mounted Floyd nuts in this video:
[youtube]https://youtu.be/uY1gzg5LoPc[/youtube]
Mentioning how the screws on top-mounted Floyd nuts can work loose over time was something I included because Ken said he has observed it. I never have. I have an ESP EII-ST2 that has a top-mounted Floyd nut. It's one of my main gigging guitars, and I have played it a LOT. The nut is still rock solid.
I also have about five other guitars with Floyds. The nuts on those are all mounted the common way, with mounting holes screwed in from the back. Those have all stayed rock solid as well.
From a theoretical standpoint, the mounting holes method seems like the smarter, more sturdy way to mount the nut. I would always opt for it, given the choice. In practice I've had Floyd nuts mounted both ways and never encountered a problem with either.