wheel spacer should be attach if have gap between base and rim molding. This will prevent your car shaking when rolling. Eventhough you cannot feel the vibration but your tyre surface will koyak at one side.
From my experience of fitting 25mm hub centric wheel spaces on stock 17 inch wheels made the car feel loose. I didn't like how it handled so took them off soon after fitting. It did however make the car look nicer. 10mm spacers might be not too bad.
Make sure you get good quality spacers like H&R or eibach pro as they are made to fit properly as long as you order the BMW compatible ones. I wouldn't get no name brand ones off ebay. You will also need longer bolts to compensate for the spacer if not your wheels will not be held to the hub properly. BWM bolt pattern is 5x120mm. Lots of cars including Audi and Mercs may have the same but make sure the centre bore is the same too. Also make sure they are legal to use where you live.
I have been using wheel spacer for both my e92 M3, the 1st M3 for more than 1 year, the 2nd M3 since I took deliver the car last month. No problem so far. Just make sure U get a good quality and hub-centric. My 1st M3 with 15 mm front and 12 mm rear with corresponding longer nuts. The car looks better and cornering IMO are better.
I have 4 x (2 sets) Eibach German made (TUV approved) hub centric spacers for sale if anyone is interested. But its in Melbourne so either wil have to wait till I get someone to bring it over there (latest by mid year) or pay for postage which might be expensive.
1) set 1 is 15mm
2) set 2 is 20mm
3) 20 extended bolts