I LOVE SPANX! They have been a lifesaver for me under certain dresses that are clingy fabric (and with jeans and tighter shirts where you don't want any bulges over top!). They can definitely get hot so I'm thinking that if i need to wear them at all for the wedding it will be for pictures and the ceremony and then off they come!
Some Spanx have a line down the middle that in very sheer fabrics (like certain wedding dresses) you'll be able to see and other more expensive pairs are totally seemless...just keep an eye out for what you're buying and what you need. Also, I find I am true to size based on the charts so go figure?!