Is is a bad idea to transport wedding cakes in Rubbermaid containers? I washed them out so I don't think they have that plasticy smell anymore. I was wondering if it was better to use cardboard boxes.

I don't see a problem with the plastic. I have special totes that I use for kitchen use. As long as they are only used for food, and always washed before each use.
Your cakes won't be in there very long, so the plastic smell won't hurt. You may just have trouble putting them in and taking them out without bumping the sides if they very high.

I prefer boxes because it's safer to get them in and out. With large wedding cakes, I put a clean sheet down in my vehicle and transport with no box. I also put several on a large pan or in shallow full sheet box.

I store them overnight in rubbermaid containers that I purchased and washed just for this purpose. I lay the top down on my table, place the cakes on and then put the box over.

When I'm transporting, I generally use boxes or just put the grip sheeting down in the back of my Durango and place the cakes in--depends on how many, how far, type of decoration, etc.

Lisa