Tell all the Truth but tell it slant --
Success in Circuit lies
Too bright for our infirm Delight
The Truth's superb surprise
As Lightning to the Children eased
“Success in circuit lies” suggests circumlocution in relation to the truth rather than telling it directly. 
 This could be positive or negative.  A politician may tell a version of the truth that is to his political advantage.  A mother, however, may simplify an issue for a small child, appropriate for their age.
Tell All the Truth But Tell it Slant