I am relatively new to the whole BMF thing, I've only done BMF on 2 cars, my 64 Thunderbolt and my Road Runner. Currently I am foiling 2 cars, 1/12 Camaro and a 40 Ford Delivery. On my Thunderbolt, that BMF is cleared over. And in my opinion, that looks best. But on my Road Runner, the paint has no clear, and no clear over the BMF. And that one looks good too.
Personally, I do my BMF after paint and before clear. I was also taught, whatever you put on the model, be it decals, paint etc etc, you should clear over it. Most of my cars, I'm not going for complete accuracy. I am going for them to look good. And for them to look good in my book, they are usually cleared and polished.
The only reason I cleared over the BMF on my Thunderbolt (and that was my first BMF job) is because I thought the BMF had a dull type sheen to it, and not really shiny. So I cleared over it and, I think it looks better. It is more shinier than what it looked like when it first was on the model.