I've done a lot of fuel trim sleuthing on my E53, E46 and my E39...The e46 folks have really done a great job documenting all this. You want to get the OBDFusion app and set it up exactly as outlined here:

http://forum.e46fanatics.com/showthread.php?t=897616

as for your specific question...here is a response on the E46 site from JFOJ who pushed the envelope on fuel trim testing:

...Real world advice from BMW and many other vehicles.

Long Term Fuel Trims are the most important value to pay attention to in addition to the engine operating temperature. Short Term Fuel Trim values are ALWAYS changing rapidly and they need to me mentally averaged. They are less of a concern or value that I pay attention to, but these do come in handy for the final analysis of a problem, usually when pinpointing bad MAF's.

These comments are ALL about LTFT values.

Anything between 0% and +2.5% is fine, anything above +2.5% should be investigated.

Anything negative (-) or above +5% is a problem in my opinion.

Anything +10% or greater will typically trigger lean codes. Not sure if +12% is the magic number, but even +10% is an issue.

Be VERY CAREFUL when using any of the "German" software tools, even BMW Scanner, along with INPA, GT1 and so forth. This does not apply to Fuel Trim valves, but where the tools describe or use the terms Rich or Lean. There is either translation problems or a different point of view as to how these tools represent lean and rich conditions. They are usually 180 out of phase as to how the rest of the world view things. So for any of the software tools that are primarily BMW specific, you have to stop and take a LONG breath and THINK when you read CAREFULLY when you see the word Rich mentioned with any code.

RARELY do engines run Rich, they almost always run Lean. Very few situations can cause an engine to run rich, the 2 most common ones are engine operating temperature too low and fuel pressure too high. Even then, these situations RARELY trigger a code for a Rich condition. You will only pick this up with negative (-) fuel trims. Also MANY of the negative (-) fuel trims are MISLEADING. Usually due to a bad or counterfeit sensor.

Also DO NOT jump into replacing O2 sensors for Fuel Trim issues initially.

If the O2 sensors have issues, they WILL NOT usually trigger lean conditions, as the O2 sensors are what actually MEASURES the amount of Oxygen left in the exhaust stream. If the O2 sensor is not working properly, then how can it measure something that is 10% out of range?