I am making a topsy turvy cake for my Sons birthday this weekend and I am trying to decide if I should do a ganache or buttercream crumb coat for the top two tiers (only the top two tiers are angled and "floating" on pvc). 

I am just thinking about it in terms of stability, the top two cakes are at something close to a 17 degree angle. 

What would you do?

I would use ganache...

It's all about your support prep.   If the base of your cake lays flat in a cut out hole....it doesn't matter the angle it was carved at.  

Thanks so much!  I was thinking ganache also.