>>339If you can find out what the hardware native DPI of your mouse is, it is best to use that to achieve the absolute minimum amount of errors in the tracking of your aim.
If you have software that lets you choose your DPI, setting it to a number higher than the native results in a subdivision of the pixels in the sensor, but it can't be perfect and will create errors. The software will try to correct for these errors, but purists like me don't trust it to get the job done right.
My G400 has two native settings, 800 and 3600. I could either use 3600 and lower my in-game sensitivity for 3D motion and lower my Windows sensitivity for 2D motion, or I could do the opposite and use 800 and raise the in-game and Windows sensitivity to reach my comfort zone. I am currently using the latter, and have great results. I might have to try the former sometime soon though.
Here's a video where a Logitech Engineer talks about sensors and tracking and stuff.