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The differential_drive package implements a simple, generic differential drive model. What the width between the wheels should be in this model depends on what shape your wheels are in cross-section. In other words, you want to provide the distance between the points on the wheel that will most often be in contact with the ground.

• The most common case is wheels that are symmetric, e.g. common caster wheels and car tyres. In this case, the wheel may be slightly rounded, so you can use the middle of the wheel as the point to measure from, or it may be flat, in which case the middle provides a good average.
• The wheel may be assymmetric, such as a wheel that is cambered in some way to give nice cornering. (I think this type of wheel is unlikely in a differential drive system, but it's not inconceivable.) Think of a wheel shaped like a cone. You would then measure from the wide part of the cone because that is the part that is most often in touch with the ground.

Because you said your wheels are 10 cm wide, I'm guessing that they are thick, relatively flat tyres, so measure from the centre.