I also refrigerate everything (I know that's controversial) but it keeps everything nice and firm and less likely to vibrate apart during transport. It never ruins the fondant or gumpaste and it keeps the buttercream and fillings firm and stable. Because even if you use dense cake, you still end up having filling that is not dense. (I use any cake pretty much). The weight should be on the supports, not on the tier below.
I don't use really wimpy fillings like preserves or mousse...did that ONE time and it was a nightmare...the cake made it, but it was bulging some and it was only by God's grace it did make it without collapsing!!!!