MINI mechanic (not dealer) tells me front rotors on my '06 R53 are worn, grooved, warped, and need to be replaced along with pads now at 3mm.
Brakes work fine - no warning lights, sounds, or vibration.

I don't buy the warped story. There is very slight grooving and wear is about 1.5mm.
How do you know when rotors that are working fine need to be replaced? Any mechanic you ask will always try to sell you parts and labor.

 

You measure the thickness of the rotor, if they are less then the value stamped on the rotor, they need to be replaced. My Mini's rotors were worn below the minimum thickness on my first brake pad change.

 

many sources explain "there is no such thing as a warped rotor" ... total myth ... so the mech' is blowing smoke up your ...... ask to see runout numbers

as Dave says the minimum thickness is cast into OE rotors .. for GEN1 front minimum is 20.4 mm (Haynes)

in the beginning years of MINI, the factory said always change rotors with pads .. and they covered this under pre-paid maintenance. As the years passed they changed their story and coverage (saving $$). I have consistently gotten two sets of pads to a set of rotors but am admittedly EASY on brakes in general .... You WILL see a slight lip .... IME the new pads are the same size and will run inside this lip which is normal. MINI does NOT recommend resurfacing or 'skimming' rotors. This is an 'old school' thing and works when rotors were THICK / overengineered. Today's rotors are relatively thin to save weight and a skim will usually bring the rotor too near minimum thickness to last the life of new pads.

when you have everything apart to swap pads you are 75% (or more) of the way to swap the rotor too .. and rotors don't cost much since you don't really need slots or holes for street use. If in doubt just go ahead and get some new (plain jane) rotors.

 

When my pads needed to be replaced, I went ahead and changed out the rotors. They aren't all that expensive and it's easy to do. When it comes to brakes, it doesn't pay to cut corners.

And as others above have stated, warped rotors is a misconception.