I usually decide mostly by how heavy it will be when I decide how to split up my cakes for travel. If you have to carry in the cake by yourself (like I do) You want to make sure its not too heavy for yourself! For a 5 tier cake I would likely do the bottom 3 tiers then the top two, depending on the size. If you do try to transport the entire assembled cake make sure you have someone to watch/support it during transport! Eep!
-- Nikki, So Cal, www.Facebook.com/nikkibelleperchecakes