I did that last trip because I was really nervous about not sitting together. It worked out great for us. It's especially awesome for return flight so you don't need to worry about checking in during your vacation !
The only time I bought it just 1 month before our flight, we ended up with low B boarding passes. It was fine and still glad I did. It's nice to not have to stress about T-24 while on vacation.
2 weeks out is usually when I buy EBCI. Because it's non-refundable, I don't like to buy it when I book the trip as many of my trips are somewhat tentative in nature. By 2 weeks out, I know I'm going. I ALWAYS buy EBCI for a trip, because it's convenient to not have to worry about checking in.