http://www.carcraft.com/techarticles/ccrp_1304_torque_wrench_testing/
Unless you're engine building and spending the big bucks to get a digital Snap-On one, I'd just use the HF ones. I have all three sizes for a net total of $24.
Test or otherwise, I refuse to believe that keeping a torque wrench set at any given level affects anything.
That's not how springs work, and not how steel, or yield points, works.
The readings changed over time, as the surfaces change and break in and wear, as the temperature and humidity changed, etc. But not from keeping the wrench wound up.
That said, I always unwind my cheap duralast click wrench, the directions say to, and waddaya gunna do... it's a bunch of voodoo, but it's voodoo I read in the instruction booklet.
The reason it's all bull is the same reason that beam wrenches generally are not considered to need much attention or concern.
Lube tech at work has one.
Plastic handle broke 1 month into him having. Came dry out of the box to.
Torque wrenches should not be lubricated, unless otherwise stated. Some torque devices need oil or grease (think X-4 perfatorque limiters) but click wrenches generally need to be dry. If you get oil in it, it should probably be washed out with a solvent that fully evaporates.
And oiling the head obviously causes a pretty high risk of dribbles getting into the mechanism - which I'm pretty sure could dramatically lower the torque required to release it, thereby potentially risking the value of any equipment being worked on, and potentially, many lives.
