Lubricating threads can lead to over-torquing by as much as 50%.
This was demonstrated on a DIY automotive show a couple of years ago -- I think it was on one of Spike TV's "Powerblock" shows.
They showed using anti-sieze altered the torque spec substantially. Then they used Automotive Racing Products (ARP) Ultra-Torque anti-sieze compound (
ARP | Official Website | ARP Ultra-Torque) which consistently produced the correct torque spec.
Why do you need a torque wrench?
When you tighten a bolt or nut, you are stretching the bolt to a certain amount of force that provides the clamping force. I removed a bolt from my car's suspension that was about 1/8" longer than what it should have been because a previous "mechanic" over torqued the bolt. Think of stretching a rubber band: Too loose and it doesn't do anything; too tight and it breaks.
Not everything requires the use of a torque wrench unless you're working with aluminum, then use a torque wrench on everything. When you're tightening a hardened steel bolt into aluminum, you're stretching the threads in the aluminum, not the bolt, to provide the clamping force.
The torque specs for lug nuts on cars can range from 50 pounds to over 120 pounds (
Wheel Torque Chart - Discount Tire Direct). Check with the car's manufacturer for the correct specs. If you over-tighten, you risk warping brake rotors or snapping off the lug stud.
Every "shade-tree" mechanic should have a decent torque wrench in their tool kit.