You can always try selecting one now, chances are that it will tell you there is a charge but if it doesn't then you're free and clear (and if it does tell you there's a charge you can just back out and no changes will have been made).

If you wait unitl check-in time and the premium seats are still the only ones available then you will get one w/o additional charges.

From my own experience if I want an EML seat, I pay for it when I book my ticket. I'm sure others do the same thing. Your risk is that someone may re-think the middle seat they have and decide to spend the money on a EML seat in which case you could wind up in a middle seat.