I’ll add a few thoughts.
It’s definitely important to keep IE7 in mind when optimizing. And if the goal is to get people thinking about the tail of the latency distribution, then yes, IE7 is the right browser to optimize for.
But the statscounter chart shows that IE6/7 populations are shrinking quickly. A year ago 35% of users were on IE6 or 7. Now 19% of users are using these browsers. In a year, if we follow the trends on the statcounter page, we’re looking at well under 10% of users on these browsers. On the other hand the footprint of modern browsers increases every day.
Ideally optimizers would look at waterfalls for both IE7 and IE8. But if you’re only going to think about one of the two, you have to ask yourself: do you care about optimizing for the majority of your users, or do you care about optimizing for the tail of your latency distribution? If the tail is important, focus on IE6 and IE7. But keep in mind that these browsers are effectively going away (look at the trends on statscounter before stating otherwise), so I would discourage fine-tuning sites for these old browsers. On the other hand, modern browser features like data URIs, increased parallelization, speculative JS fetching, are only becoming more common, so investing in them will pay off down the road as more users transition to modern browsers.