The code in mention would work for some simple cases but it overrides the default z-index of the wrapper and this might cause problems e.g in the below scenarios:

1) If you have a RadWindowManager, this could defy the logic of dynamically increasing z-indices
2) This could defy the logic behind ShowOnTopWhenMaximized property
3)Other problems due to overriding the style of the wrapper DIV while the inner client zIndex property holds the old value might arise.

That is why I recommend to use the style attribute which has additional embedded logic implemented for this particular case and this is more reliable solution for the case. 

You can find a more detailed KB article below:

http://www.telerik.com/support/kb/aspnet-ajax/window/show-radwindow-above-radmenu.aspx

Information about RadControls' z-indexes values is available below:

http://www.telerik.com/help/aspnet-ajax/controlling-absolute-positioning-with-zindex.html

I hope that the provided explanations and resources are helpful, let me know how it goes.