For what it's worth, while it is really a personal choice, I think you are being perfectly sensible. When I know that I need a trip, know the dates, and find a combination of flights/fare that I like I just book it there and then. Right now, I have trips already booked in every single month of 2016 except July! I will need a few more, and some are less certain in terms of dates so couldn't be booked (I usually prefer to book when I am effectively sure of dates and thus buy restricted tickets). if I was booking for 4 or 5, I'd be even keener to "lock" a price early.

At times, I find a new offer which would have been even better than what I booked, but I accept that as part of the risk of booking early, try not to think of it, and remember that on the whole, the generic strategy of booking early works vastly better for me.