Modoc, while cracks can result from an inadequate sub grade, they are more likely to result from shrinkage. As with many materials, concrete shrinks as it 'dries', which can result in cracks. This cracking often occurs very early in the process when shrinkage is most prevalent and the concrete has not developed any significant tensile strength. These cracks may not be noticeable but they are there nonetheless.
This is why saw cut control are generally most effective when cut with 'early entry' saws that can be employed immediately after finishing operations. Wait 24 hours and your likely to have shrinkage cracks before you get around to cutting.
Control joints direct the cracks where they won't be noticeable but they don't prevent them. Shrinkage cracks can be prevented through careful planning and execution but it's not easy and can be more costly.
Larger aggregates, limited mix water and proper curing are some of the more significant preventative measures. A willingness to pick the right weather for placement is also very important. Most are not willing to go to the trouble to prevent cracks and rely on control joints to hide them. Nothing wrong with this approach but you can build a crack free floor if you want to.
Lastly, I'd suggest that you spring for some properly positioned steel reinforcing. While it won't prevent cracks, it will hold the concrete together if it does crack and maintain the structural integrity of the slab. Wire mesh is fine but it's more difficult to support. Rebar is a little more expensive but it is rigid enough to stay put in the center of the slab with reasonable efforts to support it.
