Depends on the car. More often than not, I'll let it go all the way past redline, just shy of where it "bounces back", before I shift.
Sometimes, if I really want to push a car for all it's got, I'll take a closer look at the HP/Torque/RPM info, and change my shifting accordingly (usually means shifting earlier, and a lot of times it won't even hit the redline at all). Sometimes you can see the power band at work when you're driving, especially in the higher gears. At the high end of 4th gear (as an example, I'm not being specific), you'll see that it's not accelerating so fast, maybe 1mph increase per second. Then you shift, and let the RPM's drop back down to "optimum range" and now you're getting 3-4mph increase per second.
If I'm paying attention to that, I usually try to set it up so that the top HP occurs in mid-gear.. halfway between shifting into a gear and shifting up to the next one. Which I think would be the right way to do it. I've been doing this more and more lately in GT3, and no doubt I'll keep doing it in GT4. Takes a little more mental effort to keep eyeballing the tac and remember where you need to shift, but it's worth it in the extra performance you'll get.
This is one part of the game where I and others wouldn't mind seeing a graphical representation of the full power band, rather than just "350hp/5400rpm" that's there now. In order to actually SEE where the power is at different RPM's on each car, and adjust our shifting to match and get optimum performance.
This is also one place where manual transmission has a clear advantage over automatic.. take the above example of 4th gear.. with auto, you have to keep pushing in 4th until it redlines. With manual, you can just pop it into 5th and jump ahead.