Buy enough points that if you can't get the value rooms, you'll still get the vacation time you want. Don't skimp on points assuming they will be available and treat any trips you get in Value rooms to be a bonus where you get to bank a few points into the future or stay an extra night.
The problem people make for themselves is that they buy only enough points for a studio - and only enough points for a low point studio (BWV Standard Views are another one where this becomes a problem). When those rooms aren't available, then they need to go up a category, and are shaving nights off their vacation - and that tends to cause disappointment out of the gate. If you have enough points to be able to book a studio - or even a one bedroom once in a while - a whole world of additional options opens up - with points for a one bedroom seven month options open that are not otherwise available.
Another thing to be aware of is to own enough points that if your travel plans have to change, you can switch seasons. Only having enough points for January in a value room, and then needing to change your dates to August because your job changes and you can't go in January will change both the point cost and the availability. You are buying something for the long term.
