For my tiered cakes, I usually only ever do stacked. And I have gotten very used to delivering three tiers fully decorated and assembled. I get a large packing box (usually get them at Staples or Office Max) nad put the cake in that. So, if my bottom tier was 14 inches, I usually put it on an 18" base, then I would get a 18 x 18 x 18 packing box. I then cut the corners of one side of the box (length wise) to create a flap that I can pull down and easily slide the cake into the box. Then I just tape that side back into place. If the cake is too tall for the box, I tape up the top box flaps to make the box longer and then will cover with foil if needed (like it is raining or snowing out).
When it comes to tiered cakes that will use pillars (which I don't do many of) I try and get as much done as I can at home, then transport the tiers separately and assemble on site and finish whatevery I need to there.
When transporting stacked cakes, you need to make sure that you have a strong support system--make sure you have enough wooden or plastic dowel (or straws like sone people use) to be able to suppor the weight of all the tiers above and if using foam board or cake boards in between your tiers, putting a long center dowel down the middle of all tiers will help keep them all toghter.
I used to do the wood dowel thing with the center dowel, but I now use the Coast SPS system (similar to the Bakery Craft SPS system) and I love it! They are plastic seperator plates that "lock" into plastic pillars that serse as the dowels. Very strong. I then do what B. Keith Ryder demonstrates in is Whimsy cake instructions (http://www.cakecentral.com/article1-Instructions-For-Building-A-Whimsical-Tilted-Cake.html
) by attaching the boards of the tiers to the plate with double-sided carpet tape. This keeps everything nice and secure--similar to the purpose of the center dowel. I love this method and it works great--I was even able to transport my whimsy cake with no issues. Here is a link to the Coast SPS system: http://www.cakedeco.com/cgi-bin/webc.cgi/st_main.html?p_catid=266
Hope this helps some.
Oops forgot to mention that I have a mini van and having a vehicle that can fit the large boxes and keep them on a flat surface really helps!