OK, this is how I would handle any folded project:
First, I look at the PAGE as the trim for the piece of paper which will be folded to become the brochure. (We may think of the folded areas as pages, but I don't think of them that way.)
There are always two pages to a folder project: The front and the back pages.
I would set up my two pages as the final trim size.
Then, I would specify a bleed for the document.
I would probably use the master page to designate the fold marks (as guides) as well as safety areas for type.
I might use text frames on the master page if I needed text to flow from one folded area to another. It depends on the purpose of the brochure and the text layout.
Finally, if I needed to indicate where the folds were for the client I would specify to print guides in the PDF. Although if I had a particularly brain-dead client (I know, redundancy), I might draw dotted fold lines down the master page to indicate where these folds would be.
But that would not be what I would send to the print service provider. For them I would use the bleed and/or slug area to indicate where the folds should be.
Because the page is the size of the trim, these fold marks can float anywhere the proper placement should be. (I would have gotten the fold mark placement AHEAD of time from the print service provider. I would NEVER try to anticipate where those folds should fall by myself. I know better!)
I feel this workflow is the proper technique for a designer or production person and winds up with a document that is most adaptable for the needs of the print service provider.