With dry mounting, it is essential to make sure all things are fully dry - this includes the board(s) that you are mounting on and the one you use above the print whilst it cools.
You do not need to drive your self 'mad' with this, simply heat the board(s) with the iron and then place them on a dry place to cool down. As soon as the board(s) have cooled, then proceed with the mounting. If the room you are using is very dry anyway, store your boards there a day before mounting (NOTE: if, like me, the only place you have room for your dry mounting press is the darkroom, do not store your boards in the darkroom. Good quality archival mount boards are quite porous and easily absorb chemical moisture in the air).
Best of luck with the mounting.