The issues:
1. You can bank points up until the end of the eighth month of use year, e.g., for Dec that means July 31. Thus if you have to cancel a trip and you do it after the end of your eighth month of your use year,you cannot bank and must use points by end of year, which, of course, can be very difficult if there are only a couple months left in your use year. That is a possible scenario you face if usual time going is in last three months of use year. Thus, it is recommended, if you have usual time of year to go to get use year that avoids that issue, meaning one that begins shortly before (and no more than 8 months before you usually go).
2. Because the risk of cancellation usually results from some unforseeable event that takes place near time of trip, you also face the 30 day rule. If you cancel less than 31 days before date of arrival you cannot bank any points that had been used for that trip (even if banking date has not passed). Points in that situation are put into a holding account, must be used before end of use year, and can only be used by calling no more than 60 days out for a reservation. For that cacellation issue, you again want to try to avoid having your regular trips close to the end of your use year, and thus again having one that starts shortly before you usually go is preferred.
Other than the above, use year does not matter. If you usually go virtually any time of year, don't even consider the issue. Teachers I know usually don't go in fall but may go Christmas, spring break, or summer (and usually before mid-August). The ideal use year for that pattern is December when considering the cancellation issue (Christmas trip is at beginning of use year, spring break one is only 4 to 5 months into it, and summer trips, though later, still give you until July 31 to bank and at least 5 to 6 months for another trip if you have to cancel less than 31 days before the trip). If you are a teacher who would not consider Christmas trip and only spring break or summer, February or March would be ideal.