Using a default installation of the 3.0.2 suite, opening the Composer view(with the MapQuest OSM tiles as background) and clicking print (to pdf) to activate the print preview there appears to be a couple of issues with the scale given in the scale selector box.
One- if one slides the zoom slider so that the scale selector box gives 1:512 000, and then opens the scales list in the box and chooses 1:512 000 the scale visibly changes in the print preview map (it goes to the next smaller scale).
Two- the scales in the scales list in the scale selector box do not entirely match the list of scales given in config.yaml: the smallest (1:524288000) is not listed.
Three - there are 17 zoom levels in the default Composer map, 18 in the print preview's zoom slider, and 19 in the scale selector box.
The scales given by the zoom slider in the default Composer map roughly match the scales given in Config.yamls, i.e 1:68k something becomes 1:64k etc, but this should not be reason for this default behaviour?
I have tried changing the number of scales config.yaml as well as changed the maxZoomLevels for the map, but besides zoom slider in the print preview following the number of zoom levels (scales) of maxZoomLevels, even if the main map's zoom slider does not, I cannot get things to precisely line up.
A search here and abroad did not reveal this issue described, but if parts of this (or all) is due to a lack of understanding of certain fundamentals on my side, then pray tell where I can go read :)