First, let's differentiate between "should" and "must".
If you have an AWD vehicle, you MUST rotate the tires at an even and consistent interval, such as every 5k or every 7.5k miles. Otherwise, uneven tire wear will lead to very expensive damage to the AWD mechanism.
If you don't have an AWD vehicle (I assume that you are still driving that Kia Sedona van), then it is advisable, but by no means absolutely necessary to rotate the tires. The tires will wear unevenly from lack of rotation, but no mechanical damage will take place. What will take place is uneven levels of traction from tire to tire as they wear, and this could lead to problems ranging from difficulties in rain (hydroplaning) and snow (skidding) to poor road-holding and poor ride quality.
There are some folks on this board who state that they never rotate their tires, but the usual result of failure to rotate tires is that you only buy 2 tires at a time as they wear out. By the time that you are ready to buy the next two tires, the original model tire may not be available, thus leading to different types of tires on the front and back of the vehicle. This in and of itself can lead to handling and traction problems.
Yes, it costs a few bucks to rotate your tires, but I think that the additional safety benefits make the cost worthwhile. And, if someone chooses to not rotate the tires on an AWD vehicle, they are literally flushing money down the drain, rather than saving money.
As to specific questions regarding your own vehicle, I can't give you any better advice than to tell you to open the glove compartment, take out the Owner's Manual, and see what the vehicle manufacturer has to say regarding the recommended rotation pattern.