Do You Change Camera's Compression / Quality Level?

I ususally leave it at default, but do adjust sometimes to accommodate weaker networks or storage concerns. Most cameras I've come across seem to default to around (rough estimate) 75-85% of maximum bitrate... which interestingly, coincides with most image editing software I've used that tends to default to saving JPEGs at 75-85% maximum quality.

In most cases the compression level is set in the VMS due to the level of integration, so we set the level to what we feel comfortable with, over-riding whatever factory setting may be,

I agree with Keefe. While most of the time, the default settings are acceptable, tuning the camera quality/compression is an essential part of obtaining a perfect image. This is especially important in complex scenes such as Casinos and outdoor areas. Another component of tuning is storage and bandwidth usage. You may calculate storage using either the manufacturers storage tool or an aftermarket tool. Typically you do this BEFORE you buy and deploy your system. So, that being said you need to match your actual install bandwidth and quality settings for each camera up with your original calculations. Not doing this can not only affect video quality but can hurt retention of video and bandwidth. Now, if you don't fully understand a cameras quality settings, leave them at default or consult the manufacturer for their recommended settings.

As Luis said, the VMS will typically manage these settings so ensure all your changes are done at the VMS unless told otherwise by your VMS vendor.