Moderate risers tend to make skis feel a bit less side-to-side stable, but in exchange, they reduce the ammt of edging force you need to exert, and slightly increase the distance your knees must move sideways. Since you said that you spend 2/3rds of your time in crud, anything to increase stability probably is a good idea, so I would go with no riser.
Personally, I use no riser (other than the natural binding height) on my crud skis (10ex's, Explosivs), but do on my carving skis. I would rather have to work a bit harder to get an edge if I am on a fattie and encounter hardpack, than have my fatties feel unstable when I get into chop/crud. After all, that is the primary reason I use them. Float, per se, isn't the real issue - stability is. You can ski just about any ski in untracked, virgin soft/deep snow.
Tom / PM
PS (in edit) - Just for the record, there is one knowledgable guy on Epic who (as I recall) lives up in the Banff area and recommends risers even on fatties. He seems to be a minority of one, but perhaps he has the luxury of not seeing as much heavy wet, heavily tracked up slop as I always seem to run into.