2007 Mazda 3 front springs

I have a mazda 3 maxx 07, it over the last 8/9 months dropped on 1 side (driver side) so now has a pronounced lean but was told by mazda that they have a limit of 10mm or 10cm (cant remember) from front to back they have now taken the car in to sort this problem but only doing the side that has the lean.......i want to know for safety reasons do both sides need doing or is just fixing the spring that has the problem or should they be doing both??

regards

mark.