In the end we will all have our opinions on whether it is worthwhile to spend less at the pump, but there are a few facts which can be supported by evidence. These points all refer to the OP's specific make and model of car, the 04 530i:

1) Using lower octane may reduce performance in terms of HP/MPG. You may or may not notice this difference depending on environmental factors, driving style, etc.

2) Using lower octane will not harm your car. Remember though that as Mr. Plucinsky implied, all lower octane fuels are not created equal. The lack of sufficient fuel system cleaning additives may gunk up your engine over time. (This is why I use Shell most of the time)

3) The only way to know if you are actually saving money or not is to record your mpg on premium vs regular, and see what your numbers come out to. It may be that you waste money in the end due to a decrease in fuel economy. However, you wouldn't be the first person to claim increases in fuel economy either. On this point YMMV.

As to whether or not you should be concerned about this small savings, that is a matter of opinion. If $3-$5 will make you or break you, you have bigger problems when servicing comes due. Having a nice car and a lot of money does NOT mean you shouldn't try to save money at the pump. In my opinion, it is wise to save money in things large and small.

 

Adding something not mentioned yet to the thread - I personally use regular ("95" here, probably "89" stateside) in winter because (a) my tyres won't let me go over 130mph anyway, (b) they lose traction so much more easily that I hardly ever set off in 1st gear, (3) the roads are generally slippier so those extra BHP from a premium fuel really aren't coming into play, ever. Now I'm on the summers and the temps are well above freezing, now it's time for the top fuel.

 

It wont hurt your car. The power will decrease since the ecu will see knocking and retard the timing of the ignition and possible increase the duty cycle on the injectors. In the end you will get less gas milage and less power by running the lower grade. Here (Pheonix) it's .20 more for 91, that equates to $3.00 more a tank. You really are not paying all that much more, rent 3 less redbox movie that week, lol.

 

Ironically, given that the $.20 difference between premium and regular seems to remain constant, as the price of gas goes up premium becomes a better value!

 

Lets just tag this thread as Religion / Politics / 7500 oil changes. Who is correct?
Here is a summary. Run what ever gas you want. If your car is damaged or not, you are putting in the gas which you feel comfortable putting in the car. Fill it up and drive.