I don't think tires themselves will change their balance much. Once formed, assuming the wear is even with time, they will stay very similar in their balance. As others have said, it's the external things like weights, etc that might change. But even though shouldn't be changing by the week.
I cannot say for sure, but I can think of another potential cause of perceived "out of balance" could be the tire is getting "set" from sitting overnight and goes slightly out of round when cold as you first drive it. This is often referred to as flat-spots, but they really are just forming to the loaded side of the tire on the ground as they cool from the last drive cycle. But the way you describe it, they are OK for a few days, and then misbehave. So my question would be: how often are you driving the vehicle, and how far between uses? This happens to me at times as my vehicles often sit for random lengths of time between drive cycles. I have to drive them for about 10 min for them to smooth out.
