I am currently stuck with blender 2.8 (I know that is a beta right now but I remember having the same problem with 2.79 and I finally want to solve this).
I am trying to make an object a child of a bone. To do this I select the supposed child object then shift select the armature (that was made using rigify), switch to pose mode, select the parent bone and parent it with ctrl+p to the bone. When doing this the (carefully aligned) child objects is rotated and moved and I can't figure out why. I am sure the must be a way to do this without having to reposition the child after parenting.
I have applied all transformations to the child object and also checked to have reset all transformations on the pose.
Please check the images to see what I am talking of. Thanks for all the help you are providing and all the work done on blender.