I'm sort of confused by your question. Are you saying you want to cut slabs 6" wide and 6" thick and you are saying this because you don't want to waste wood with the kerf width and the idea that the wood will dry straighter? I'll answer based on assuming these are your questions.
Waste from the kerf cut; If you are doing the initial milling with a chainsaw than yes, cutting thicker slabs and resawing later on a bandsaw is a good idea. With my hardwoods I routinely saw are 9/4 or 10/4 for drying and resaw in my shop. It is sort of a middle of the road approach since I chainsaw mill. I save wood but I'm not leaving the slabs so thick that they take forever to dry. Tony is correct above, you produce more product by resawing later. Also I believe the size I'm slabbing dries straighter. Leaving slabs 6" thick is really going to lengthen your drying time significantly, 6" is just too thick to be a good idea.
As for the width, the best answer to that is that it all depends. It all depends on what you are building, how wide is your jointer and/or planer, and whether you want flatsawn wood or looking more for quartersawn wood. In the end it really doesn't matter that much in regards to width since you can always rip to the width you want on the table saw. For me, a deciding factor is the width of my jointer, mine is 8". To flatten boards I need them at 8". If I want a wider panel I'll take a wider board and rip to 8", flatten on jointer, plane and then reglue boards together as they were ripped, often blending the grain and figure so you can barely tell they were ripped and rejoined.
So here are my random thoughts on width;
- Cutting narrower boards will not speed up drying significantly
- Cutting wide boards may cup more but that can always be addressed later on the table saw
- As Tony said, cut to the width you can best handle is a good idea also
- The logs you are talking about are not wide enough to quartersaw. Quartersawing gives you more narrow boards but boards that are more stable.
- Flat sawing a log gives you a mixture of cuts and you will get quartersawn lumber with your middle cuts.
- I like to edge one side or both sides of my slabs to make it easier to cut for width and to edge if I'm resawing on my band saw. I don't do much natural edge stuff.
So, I'd cut thicker slabs, but not too thick, look to resaw later and not worry as much about the width.