We realize it could be the monitors and working to get calibrators to compensate. The solution to that is to zoom in quite a bit and if the banding is still showing then its in the file. We do not have any kind of proofing system and printing will assume the file is correct and run almost the entire job before noticing there is a problem. It still is like throwing darts in the dark.
The other thing that likely was causing it was that the RIP color profile was different from the software profile. We have now standardized both to Adobe U.S. Web Coated SWOP v2. This has helped, but there still seem to be situations where despite out best efforts the banding still shows.
As far as gradient color we generally do not mix colors. We will create a gradient with one side 100% fill color and the other the same color set to 0. If another color is needed a second background object is created with 100% of the second color. This has worked decently, but not always.
We generally use the default preview, but I have been working in Overprint Preview because we will get customer files with transparencies that are not supposed to be there, but without OP view turned on you can’t see it and then it prints incorrectly. This has been real problem lately.
As for placed images they generally are right in Photoshop and then embedded so there usually isn’t a problem. Although if making gradients in Photoshop similar banding issues occur and then you have to sort that out as well.
And we ALWAYS work in CMYK, and work in the highest bit depth our system will allow.
What I don’t understand is Adobe used to have a method to assign how many steps you want in a gradient fill which eliminated banding altogether. This is no longer available so you have to stop and figure out which needle in a haystack is causing the problem. It should NOT be this difficult.