One thing to keep in mind about crossing straps in the rear is that the angle of the strap to the direction the load travels reduces the effective rating on the strap. using the same 45 degree example, if the straps are on a 45 degree angle to the direction the vehicle will travel in a frontal impact then they have only about 70 percent of their strength. Say you have 20k pound straps that are a few years old so we'll say they are good to 18k and then you put them on a 45 degree angle and now you have a strap that is good for 12600 pounds, times two and you're around 25k pounds holding a 5k vehicle down and as soon as you add in some inertia and acceleration you dont have much room left for safety factor.
The one thing I'm afraid of while towing is getting into a frontal impact and the rig coming into/over the tow rig because the straps breaking. If you do cross just one end, cross the front so the rear straps have as much strength as possible.