I have the DualEdge deckling tool, which is nice, but I actually prefer the look of a simple torn edge using a straight edge like a long ruler. Tearing from the back side seems to work better, but with either tool, trying both front and backside tearing-the looks lill be different. A "loose" tear, where I don't try to follow the straight edge perfectly, is also worth trying; sometimes the result can be more natural.
Different papers will give different results and since paper has a grain direction, tearing cross-grain may give different results than tearing with the grain.