Starting with an asset (static mesh for instance) with a precise rotation value (such as a whole number) in the details panel will result in a imprecise, slightly fractional off value when rotated using the transform widget.
In other words, if you start at a rotation value of "0" and rotate the object "90 degrees" the resulting value in the details panel will be fractional: 89.99985
This is worse with grouped objects (-89.999962, for instance)
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-28883 in the post.