Choosing the correct trust values can be a complex process.
It is not enough to consider one system in isolation. You must ensure that the combinations of trust settings for all of the source systems that contribute to a particular column produce the behavior that you want. Trust levels for a source system are not absolute—they are meaningful only in relation to the trust levels of other source systems that contribute data for the trust-enabled column.
When determining trust, consider the following questions.
Does the source system validate this data value? How reliably does it do this?
How important is this data value to the users of the source system, as compared with other data values? Users are likely to put the most effort into validating the data that is central to their work.
How frequently is the source system updated?
How frequently is a particular attribute likely to be updated?