According to Qt docs, The selection behavior of QTreeView can be controlled by the show-decoration-selected property. But setting this property in style sheet has no impact.

It can be reproduced easily with "Simple Tree Model" example and setting stylesheet as follows:

setStyleSheet("QTreeView { show-decoration-selected:1; }");

Changing show-decoration-selected to 0 does not make any difference. Looks like value specified by the current style is not be overridden by the style sheet.
For the styles Windows and WindowsVista the tree decoration is not be shown selected by default. And for Fusion style, Its is always shown. Cannot be changed by the stylesheet.