I would not think altitude would effect it and here's why...
Water is not a factor because there is basically no water in chocolate(very very minimal amounts)....so that isn't effected
The temperature is low and does not require boiling...boiling is usually when those temperatures are changed...candymaking for example...you have to know the differences in temperature at different altitudes or you are in a world of trouble...
I do not see anything in chocolate that would make it so the temperature would change in the tempering process...the crystals in the chocolate, as far as I know, are not effected by altitude and that is what tempering is dealing with...
Robert
www.chocolateguild.com