Using your tachometer for shift points is pointless in these cars. If you drive it long enough, you'll learn by ear & feel when to shift. If you're driving hard, your eyes need to be on the road (did I say road? I meant track) all the time. The reason the tach is pointless (assuming you have an ls1 car with a 6-speed) is that the GM tach is really lazy. You'll be hitting the 6200 rpm rev limiter while the tach shows less than 6000. This is under wide-open throttle conditions. If you want the added safety of an external shift indicator that won't lag, I suggest a shift light. You can set one to come on at whatever rpm you like (5800 is the number I've heard most often, as this is where that REALLY wide ls1 powerband peaks), and it doesn't lag like the GM tach. For more information, check out the site below. This guy really knows what he's talking about, and has lots of details about great ls1 mods on his site.
http://www.xse.com/leres/ss/shiftlight.html