While swing speed is a good general guideline, keep in mind that there isn't a standard definition of various flexes -- while they're generally in the same neighborhood, borderline cases may differ. For example, when Mizuno suggested shafts for me, I was suggested a soft-stepped stiff in two brands, but a straight-up stiff in a third (the difference in shaft weights on the suggested ones also fell into this).
Furthermore, there are other factors that tend to affect the 'right' flex.
Certainly, looking at swing speed with any chart is better than picking blindly ("well, Tiger uses X100s, so those must be right for me too!"). But there's more than just that in deciding the right shaft.