I know couples who started out mono and then went poly, and couples who started out poly and are currently mono...But it's best to be honest about what you want when you're first discussing possibilities.
At this point in my life, mono really isn't an ethical option for me, so I'm upfront about that. Theoretically, if I had no on-going emotional entanglements and was going to just start completely fresh in a new relationship, then I would have more flexibility, and might just hold off on bringing up mono/poly until it came up naturally in conversation, at which point I'd be honest about my history and my attitudes, but wouldn't automatically rule out the possibility of mono. I consider myself polyflexible, able to be mono or poly depending on the circumstances.
I don't know anyone who tried playing like they were interested in mono when they really wanted poly and then sprung it on their SO once the relationship was solid- but I can think of a few people I suspect secretly wanted mono going into a relationship with someone who was not mono and had no real interest in being so.