I try to book as far out as I can, but seldom as far as 300 days; maybe closer to 6 months. I have the luxury of flexibility (and miles in several FF programs), so I can usually find something that works for me. I monitor availability and adjust my travel dates as my plans firm up and availability opens up (often only within a week or two of the flight). There is typically no charge just to change dates.

When I reserve an award ticket I always know that, for some reason or other, I may need to cancel it. If that happens, I just pay the redeposit fee and consider it to be akin to trip insurance.

Just an aside. I tend to "purchase" award tickets as 2 one-ways rather than a RT. This allows me the added flexibility to change one leg without affecting the other. The downside is that if I cancel the whole trip I have to pay double.