It depends how certain you are that you are going. If you have the time off for the trip and have every intention to go, I say go ahead and book it. The further out, usually the better fairs, obviously there will be some exceptions, but for the most part this is the case.
It can be costly to fly internationally, so, you have a few options. First, you could buy a ticket that is refundable. Meaning, if you cancel, you may only have to pay a fee, which isn't fun, but still a lot better than losing everything. You will pay a little more for this ticket, but worth it if you are worried. It can be hard to find refundable tickets, but they are out there.
Finally, a big thing to consider with any trip, especially with air, is trip protection. It will range in cost depending on the cost of your trip and such, your travel agent can help you with this if you use one to avoid over-paying on your coverage. This way, if you have to cancel and the reason you cancel is a reason that is covered, i.e. illness, etc. they will cover the tickets or any costs that you lose. It is a really good thing to have when traveling.
Hope this helps!