You gotta look at what RPM's you are at in the next gear when you shift, it's more important than how much you're making in the current gear at those RPM's.
My car starts dropping off after 6600 RPM's, but I still rev to 7k before shifting because I have about 40 more rwhp in the next gear than if I shifted at 6600.
If you really want to tweak your drag racing, use a gear calculator and your dynograph, and figure out the best shift point for each gear.