In many cases, if a cake is going to get messed up it's the box that will do it (or misuse of the box by the customer). If you decide that a box is necessary, Bakery Craft delivery system boxes are awesome.
http://bakerycrafts.com/Home/Items/tabid/69/SrchStr/del%20sys/Default.aspx
(scroll to the bottom of the page).
I order samples from BRP Boxshop...as a hobby baker I can't justify ordering 50 or 100 but for $10 I can get 5 boxes of various sizes or cake boards..
Just an idea. I like having a box though.
I use and re-use brown cardboard boxes for my extra-large or oddly sized bigger cakes. I used to line the box with white butcher's paper, but don't even bother with that anymore 
Another 'trick' I do for 12 inch or smaller two-tier cakes is to buy extra deep pizza-like boxes with a swing lid, put the cake in and then and staple them half-open, then I use a clear cellophane to cover the gap. These boxes are cheaper for me to buy than the all white bakery boxes (although I have too many of those too!)
I never box tiered cakes, but I always provide the customer with a bakery box or two, just in case there is any left over cake. I have never had anyone complain.
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