You'll get varying answers here.

Personally, I almost always do all 4 rotors and pads whenever I purchase a used BMW. Then I don't worry about them again. For example, my wife's 2010 x35d we picked up had the fronts done... but the rears are down to something like 3mm. I'm going to do them all (+ fluid flush) and call the brakes done for the period of ownership we have it. Now, this is easy for me to say because I do my own work and it would only set me back about $300 all-in. I'm sure your dealership will charge you at least $500-$600 just to do the fronts.

So, your decision is going to be based on your situation. If you want to just do the pads... you should be okay. On a BMW, I've never done pads without doing rotors so I don't have any personal experience with this. On my GT mustang it was fine though.

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2005 X5 4.4i Build 04/05 Maintenance/Build Log
Nav, Pano, Sport (Purchased 06/14 w/ 109,000 miles) (Sold 8/15 w/121,000 miles)

2006 X5 4.8is Build 11/05 Maintenance/Build Log
Nav, DSP, Pano, Running Boards, OEM Tow Hitch, Cold Weather Pckg (Purchased 08/15 w/ 90,500 miles)

2010 X5 35d Build 02/10
Nav, HiFi, 6 DVD, Sports Pckg, Cold Weather Pckg, HUD, CAS, Running Boards, Leather Dash, PDC, Pano (Purchased 03/17 w/ 136,120 miles)