It's a value judgement.

What are the tint values of the cyan and yellow?

Based on that, which would be the most distracting:

• A green outline around the whole object?
• A possible white sliver on one side of the object and a corresponding green sliver on the other side?

The general rule is to spread the lighter ink into the darker.

To see what Illusrator's default settings would recommend:

1. Set View>Overprint Preview to on.

2. Draw two overlapping circles. Fill one with your cyan value; the other with your yellow value.

3. Group the two paths.

4. Effects>Pathfinder>Trap. In the dialog, set the Thickness to something ridiculously huge, just to make it easy to see. Turn on Preview.

Assuming your original percentages are 100% each, you'll see that AI's built-in Trap effect would spread the Y into the C, but with a reduction in percentage(s). The principle is to reduce combined percentage so as to approximate the overall tonal value (lightness/darkness) of the darker color, making it less obvoius.

5. Object>Expand Appearance. Ungroup a couple of times.

6. Select the trapping object and check its values. You'll see that spreading Y is not 100%, but 40% (middle screenshot).

Now, I don't know about you, but to my eye, also reducing the C to, say, 70% is much less obvious than the results of the Pathfinder Trap effect (bottom screenshot).

As with autotracing and blend settings and color calibration (so-called "color management" as opposed to color correction) and other things, a little bit of human discernment (laced with a healthy measure of scepticism) goes a long way. Blind reliance on auto features often yields inferior results to a thoughtful, but common-sense manual override. So pay attention to the various settings in the Pathfinder>Trap dialog and experiment with them. This can help you understand the principles at play. Then don't be afraid to use your own judgement.

JET