I think it just really depends on what time of year you are going... if it's during a school break time it will be more difficult than when the kids are in school as less people will be trying for those times.
Also, in my checking the website off and on, I have noticed that it seems to me that weekends tend to book up more solidly (like Friday through Monday). Not sure if thats local DVC owners, but perhaps... so in that case it might be better to start your reservation mid week.