Hi there i use Vallejo glaze medium for all my washes and ink shading as it stops that glossy look you get from inks when it drys. and yes you are right i does help stop the break down of thin downed paints but if you use to much of it in your mix it does put a grey tinge in your colour.
Hope that helps you little bit as i use Vallejo paints all the time so any question feel free to ask me and i will do my best to help