The manual says, correctly, that feature values are Booleans, and thus either "true" or "false". However, the various files opendkim/opendkim.conf* use "Yes" ,"yes", "No" and "no". None of them are Booleans, however. Please fix it, and make sure that the daemon returns an error for anything different than "true" and "false", small-letters.
http://opendkim.org/opendkim.conf.5.html says:
DESCRIPTION
For parameters that are Boolean in nature, only the first byte of the value is processed. For positive values, the following are accepted: "T", "t", "Y", "y", "1". For negative values, the following are accepted: "F", "f", "N", "n", "0".