There is probably a very small risk that you will be blocked out from reserving Dec 23 and forward if you go on line at 8 a.m. eastern, Jan 23, and attempt to reserve. Morever, that small risk potentially applies only to studios and 2BRs in boardwalk or standard view; if you are seeking a 1BR in any view or are fine with pool/garden view there is no risk at all to waiting until Jan 23 to make your reservation. In fact, those are likely to be open even weeks beyond Jan 23.
The concept of walking is to reserve, at 11 months out, as many nights as you can starting before your arrival date and capture the night of your arrival date in the reservation by having a departure date of Dec 24, which prevents the particular room space applicable to your reservation from being reserved by others for your arrival date and beyond. You then call daily and drop one night at the beginning of the reservation and add another at the end until you have what you want, and actually you could probably just wait until Dec 23 and call and drop nights before Dec 23 and add up to six nights after Dec 23 (and then call Jan 25 and add your last two nights).
A secondary risk you can have with walking is actually finding a date to begin. For example, the ideal walk for you would begin with reserving seven nights, from Dec 17 to 24, on Jan 17. But an issue you can face is that Dec 17 itself may be gone when you attempt to reserve on Jan 17, thus preventing you from reserving anything on that date. That is why some will start walking even long before their arrival date (sometimes months before) and just keep dropping and adding nights until they reach where they want to be. Personally I think your chances ar high of getting what you want even if you just wait until Jan 23, but if you feel walking is needed I think the odds are very high that you will find at least one day to start walking between Dec 17 and 22.
As to what to expect Christmas time, you will find the entire World decorated to the hilt, with parades and stage shows given Christmas themes. As to crowds, you will find the largest that you have ever seen at WDW. One way to describe it is picture yourself standing at the base of Main Street at noon in the middle of Christmas week, looking down toward the castle. Between where you are standing and the castle, you will not see the sidewalks, you will not see the street. What you will see is a sea of bobbing heads all next to each other moving en masse, some away from you and some toward you (see, for example, photo here:
http://www.chipandco.com/wp-content/uploads/2011/12/crowds.png). Stand-by lines for some popular rides like Soarin' and Test Track can get to over 3 hours. Even Swiss Family Treehouse can have hour long waiting lines. You need to use fastpass wisely, don't expect to "see and do everything," and make any buffet or table service restaurant reservations when you can start to do so 180 days out.
Bill
