This issue has been closed as 'Won't Fix' due to an extended period of time without updates. If this issue is important to you please let us know by posting on the AnswerHub or UDN, and Epic will re-open the ticket for further review.
When switching editor modes Realtime rendering will always switch back to enabled unless you are in "Place" mode. While in "Place" mode the editor will remember your setting for Realtime rendering in viewport.
Also note that Geometry Editing and Place mode Seem to be tied. If you disable realtime rendering in Place mode Geometry Editing mode will also be disabled.
Realtime rendering is enabled
Realtime rendering is disabled
Head over to the existing Questions & Answers thread and let us know what's up.