After learning the hard way, I always buy it now. I also always try to buy within the timeframe that covers pre-existing conditions. For our upcoming trip, we have spent a substantial amount of money and the extra coverage I obtained (in addition to the very minimal coverage included with the tour) was well-worth it if something happens to one of us, someone at home, travel delays, lost bags, etc.
And the real bummer I have right now, is that I am worried I might have to use it for my travels. My father was admitted to the hospital on Friday and has some pretty serious issues going on. We will know more tomorrow or Tuesday, but we travel on Friday and I am just not sure which way this will play out.
We tend to use Travel Guard when we get coverage. And I usually have the travel agent I'm booking with get me the info and quote. For this tour, I used a local TA not connected with the tour because the tour group did not offer any additional coverage.