Depends on the car. Here's a few different examples of what my friends have done. I'm also researching the topic... I'm about to put a 75-100 shot on my VR6.
LS1 firebird.
He doesn't spray off the line, but it kicks on at WOT and 3800+ RPM. Needs slicks to keep from spinning. His friend sprays off the line, but a bogging mishap with a wet shot caused the fuel and NO2 to back into his intake manifold... kaboom.
H23 prelude
Just uses it as a boost in 3rd... he hates his 3rd gear.
-I'll probably use it at the top of 2nd and through 3rd, since I have tall gearing and the torque drops off fairly fast above 5000 rpm (7k redline) in stock form. It all depends on your car.
I'd get it dyno'ed. Look at your torque curve, and make decisions from there. I don't recommend first gear (unless you're wearing Mickey T's slicks like the LS1) since you'll likely just spin. Other than that, treat it like a "instant turbo"... only spraying at high RPM.
*-* Be sure your safety systems are working. 1st lean condition on a sprayed motor = boom. Set your cut-out limiter a safe distance from your redline, so you don't bounce off the rev limiter while under nitro. Respect it, and you'll love it.