Alt-tabbing to the Places panel frequently is cumbersome (need to press Alt+Tab multiple times) and unpredictable (depending on the number of elements opened the number of needed Alt+Tab presses changes). Moreover there is no configurable shortcut for focusing on the Places panel. I suggest to move focus automatically to the Places panel after pressing F9 (default shortcut for hiding and showing it) just like it's done for Ctrl+F for searching or Ctrl+I for for filtering, where the focus moves automatically to the search filed and filter field respectively.
I've added this information to an older bug report asking for the same shortcut: https://bugs.kde.org/show_bug.cgi?id=455688 *** This bug has been marked as a duplicate of bug 455688 ***