So I am working on a two tiered cake for the weekend and I'm curious-do you guys use a cardboard round under your bottom tier in addition to your main decorated board? If so, what is the best way to secure the two? I'm thinking without the round under the bottom tier I will have to place my cake on the decorated masonite board before covering and decorating which could get messy. Thanks for your input!

Sometimes I do and sometimes I don't.  When it comes to fondant covered cakes I leave the cardboard and secure it to the main decorated board with duct tape.  I just find it easier to move a cake with a board under it then trying to remove the cardboard and then placing the cake.  For buttercream I place it directly on the decorated board, slide pieces of wax paper under the cake to protect the board and frost away.

you can adhere it with a blob of buttercream smeared around -- yes i use boards under all the tiers -- 

I agree with K8.   I have often used icing as the adheisive but sometimes used just masking tape loops - so it is double faced - stickes to the masonite board and the cake cardboard. 

Yes, I do this on all cakes. A few squares of Double sided 3m white mounting tape secures the cardboard to the plate. I use it on top of my SPS separater plates as well,helps with more secure transportation.