I agree with Davidt. You may as well pick up good habits now, even if you are renting (not owning) a foregiving plane like the Warrior.
Mixture leaning is especially important when operating aircraft with engines designed for 80/87 fuel (if you know where to get this, please let me know!). See generally
http://www.lycoming.textron.com/supp...ers/SL185B.pdf
No, you don't have to have a CHT/EGT indicator to lean a non-turbocharged engine safely; although it is certainly more efficient if you do happen to have one. If your usual airplane is fitted with an EGT gauge, see
http://avstop.com/AC/2-18.html
When mishandled (which is not limited to aggressive leaning; it also includes "set at full rich always"), the mixture knob can lead to fouled plugs, increased fuel burn and expense, burned valves, and possibly a fuel exhaustion accident. Well worth getting some proper instruction. If your current instructor says that it's really not worth borthering about (below 3000' or otherwise), get a more experienced instructor.
In the meantime, here are some on-line articles worth reading:
http://www.foxflying.com/EngineLean.htm
http://www.cyberair.com/tower/faa/ap...3/0978-13.html
http://www.lycoming.textron.com/supp...ns/SI1094D.pdf
http://www.flightsafety.org.au/articles/l0053.php
http://www.eci2fly.com/Tech_Ref/Serv...ons/93-6-7.pdf
http://www.avweb.com/news/columns/182084-1.html