Interesting question. My first instinct was to answer that no, never. Then I realize that while 28mm are technically 1/56 models I find visually more "right" to use 1/48 vehicles than 1/56. At least, with modern figures. Once in the base, the extra height make 1/56 scale models to look too tiny. So, yes, I will use different scales in the same game, ocasionally.