I'm curious what the source is that 100 mg of sertraline is the most effective for anxiety? My psychiatrist, who is a very good doctor, told me that lower doses tend to be more effective for certain anxiety disorders (esp. panic); higher doses are more typical for OCD and major depression. According to him, patients with panic and PTSD often find satisfactory effects from 12.5mg-50mg of zoloft (or low doses of comparable SSRIs and even TCAs), and these are the groups that tend to be most medication sensitive.
It's very much a matter of an individual's body chemistry. I had a jittery, nauseous feeling the whole time I titrated up on sertraline; we stopped at 62.5 mg, which was my dose for a solid 4 months. When I titrated down, the jitteriness went away. A lot of the studies for sertraline have high doses, but they are paid for by the drug manufacturer, and selling more of their drug provides more profit. Consult your doctor and do what's best for you, more isn't always better.