I imagine it wouldn't have any ill effects on the transmission....but I am not an expert. If you think about it, it is almost like doing a double clutch in a manual transmission. You put it in first throw it in neutral then put it into 2nd, than throw it in N again, and so on. That doesn't hurt the car. I don't see why an automatic would be effected by putting it in neutral when stopping or going down hill. I would agree with everyone else that it wouldn't make that much of a difference, and the irritation wouldn't be worth it. The best way to save gas is to drive slower. I used to drive 85 on the interstate. I started driving 75 now. It makes a difference. If prices keep going up I am going to put it on 65.
If you really think about it, you don't get there a whole lot later.