The tatkal starts at 8AM at counter as well as online( only on irctc ).
Well the irctc website hangs for most part of the first hour. So that might be a problem.
If your train has availability in tatkal till 9AM then you may book it online for comfort.
Else both ways its a risky business. You will be required to be among...
more... the first in the queue at the counter for which you might have to reach the counter @6AM. It depends on the place you live.
In case of irctc if you have a fast internet then you may give a try but there is absolutely no guarantee..