I haven´t had any problem with changing the wheel size on bikes that I have had.It´s usualy enough that you can show that the bike type has come with bouth 17" or 18" rims depending on the model/year.Since an XT and a Tenere are basicly the same I never had any complaint´s from the Mot guys or the police. But that is of course diffrent from country to country.
To change to 18" is absolutly worth doing if you want to travel with your bike,18" rubbers can be found in most places.
Places to be and things to do, see you all out there!!