Very much each to their own on this one. Some people would prefer the flexibility of booking later but this may mean less availability. Personally, I tend to book well in advance, often at the end of the summer for the following year, and this is not just to secure reward availability. However, I always know when my holidays from work will be so not a problem with needing to get the time off later.

You may wish to consider what flexibility you have with these tickets and/or what the change fees may be if your plans were to change.