Chair Railing, what to do when butts against casing....

A chamfer would work just fine here, as would planing down the back of the rail over a few feet so that it meets flush or below the door casing. A few blind kerfs might make it bent to hug the wall better.

Mitering is my standard way of doing it. If it were new work and you were installing all of the room trim fresh, is to build up the door and window casing with backband, although that's a lot more work and likely unwarranted if you're just installing chair rail against existing trim.

Yea, just installing against existing. Will do the mitered returns, sounds like the way to go, and I'll let the tip of the chair rail touch the casing and that's where the return will start. Tnx for the advice folks

not knowing the exact profile of your chair rail I will not suggest a return or miter. I usually sand the edge that sticks by the casing so it is rounded, it looks much better than a return. a return usually makes the bottom edge short.