What kind of policy do you have, regarding your clients providing a cake topper? Do you have something written in your contracts regarding no responsibilties on your part for the topper, or damage to the cake due to the topper falling, breaking, etc...?
Reason I'm asking... I do cakes (sub-contract) for the venue I work for. Clients are provided with a basic buttercream wedding cake. I provide silk floral toppers included with the package. However, they often want to bring their own topper... some of which are NOT technically a cake topper!!!!!!! I had a cake Saturday, and the mother of the bride said she was bringing the topper. She shows up an hour prior to the wedding, with a PORCELAIN FIGURINE of a bride and groom, that was 6 1/2 inches tall, and probably weighed 3lbs. I told her it was incredibly heavy. However, she insisted...So we put it on there, and it looked fine. Well 30 minutes later, it tumbled down, smashing the edges of each tier, and broke the head off of the bride... The mother fussed that it was very expensive ($150!)... Not my problem... I warned her!
So, from here on out, i'm going to include a clause regarding toppers... If I dont provide it, its not my problem!

On my FAQ page and in my conversation with the bride, I warn them about this very thing. Here's the wording from my FAQ page ... I use the word "engineered" frequently and intentionally. I want to convey the idea that you can't just plop one up there!

What do I need to know about the cake when I pick out my topper?
You need to know the size of the top tier of cake. A decorator tells me the story of a couple who picked out an 8" antique car style of topper to put on a 6" cake. Most importantly, you need to tell your cake designer what kind of topper you are planning. Just because a piece is made to go on a cake, doesn't mean that it's MADE to go on a CAKE. Many toppers require some specific engineering structuring to be able to hold a heavy or not-evenly-balanced topper. Many a bride has been forced to place her topper on the table in front of the cake instead of on top of the cake because the decorator was not informed of the style and the cake was not engineered properly to hold the heavy ornament.

I require them to bring me the topper before the wedding so I can be assured I have the right support in place to hold it up.