Grain has too much sugars which drafts don't utilize very well. When I first got my draft everyone talked about Dr. Beth Valentine's draft horse diet and then I got the book she wrote. Drafts need a high fat diet. She also indicated when I contacted her that prevention is the better route. So feeding a high fat diet even when your draft doesn't have EPSM could help if as a preventative measure for developing signs.
Better safe than sorry. I've been feeding a high fat diet since I got him a 2. An acquaintance always poo poo'd the high fat diet and her draft came down with EPSM - they weren't able to save that horse but many horses can be saved even if they start showing symptoms and switch to a high fat diet.
I know there are draft owners out there that don't believe in the hype but it's better than not doing it as far as I'm concerned.
You can find more information here.
Rural Heritage Vet Clinic - EPSM in Draft Horses