Glue? How to attach aluminum skins....

Working with a supplier for all of my teardrop parts....I am curious about attaching the aluminum skins (using .063 to the sides, and .040 for the top) to the 3/4" Baltic Birch sides. Supplier says to let them "float" with just the aluminum extruded trim holding them on, but I've seen some say that you should glue them down. Now, I am really concerned about the difference in which the aluminum will expand/contract vs. the 3/4" birch......what do you guys think? I plan to post some pics of the progress so far soon! 
