My EM has been REALLY helpful with the room block. My mom has changed her mind about what hotel she wants to stay in - she was set on the Coronado until she realized that the only sit down restaurant there serves "mexican" food (she's a super picky eater), so now she's leaning towards the Contemporary (plus in late April next year, the price difference is only about $60 per night, so she's decided to splurge). Of course now she's torn between the two hotels, so our EM has both on hold for her and my sister for the week. She said once we do our planning visit and people start to book, she'll just cancel the extras that we don't need.
We'll also probably end up well over our 25 nights, so my EM told me the same as another poster said - they'll get the discount at one of your 3 hotels (others resorts won't get discount) as long as there are discount rooms available in those resorts. I was told that it works like discount fares on an airplane. There are only a certain number of discounted rooms available for any one night, and once they're sold out, they're sold out. The reason for the room block is to hold some of those discounted rooms, but by no means are those 25 nights the only ones available to you and your guests at a discount.
For my room block (just did mine in July) I reserved our room at the GF

, a room for my mom and dad, a room for my sister and her DH and kids, a room for my other BM, 1 room each for my two good friends that are coming, and one room for my DF's dad and brother. Those were all people I could ask - Where do you want to stay and for how many nights. That way I knew what I was booking and wasn't taking a shot in the dark.
I figured once most of them book - they're all planners like me, so they'll book as soon as they get the save the dates - then I'll add a couple more rooms - 20% more than whatever we've booked already, so that we're not on the hook for any more rooms if no one else decides to stay at Disney.
Anyway, I hope that helps. My EM told me that they really make it seem more difficult than it is - start with enough to cover the people you KNOW will book, and add on from there later. The only way that you have to be really concerned to try and hold enough for everyone so they don't get locked out of the discount is if you're planning your wedding for a really busy time of the year, and in that case, make sure everyone knows they need to book early or risk paying full price.
Good luck!