Chandra and the word may

[Imdowd80]

So this question came to me after seeing the wording for te new Chandra's ultimate. It says you may cast without paying the cost of exiled instant/sorcery copy 3 times.
So does that imply that I could pay the cost of the card if I wanted to, that way my {fireball} wouldn't go to waste

[Keyeto]

You have the choice to cast the copies, or let them fizzle the next time state based actions are checked. If you cast them, you'll have to do so without paying their mana costs.

[Birdbrain]

If you had cast the molten rebirth card from exile, then flipped the coin three times, and won at least one of the flips, could you put it in your hand? Hold on, I'll link the card

http://mythicspoiler.com/m14/cards/moltenbirth.html

[TheRagingMage]

If you had cast the molten rebirth card from exile, then flipped the coin three times, and won at least one of the flips, could you put it in your hand? Hold on, I'll link the card

http://mythicspoiler.com/m14/cards/moltenbirth.html

Yes that's what the card says
Oh, wait, can you return a card to your hand if it was never there in the first place?