The sets that we have here are the ideal sets, which include preferred Natures, Items, etc. Of course, without breeding, it can be somewhat difficult to obtain the most desired nature. Lax isn't really something that's great (since it doesn't boost anything if I remember clearly). Ideally you want a Careful Nature, in order to maximize your Special Defense.
However, these are just standards. You are free to create a Bulk Up Scrafty with whatever nature you desire. I personally wouldn't run a Lax one though.