There might be someone else with a better solution.
My thinking is that you will have create logic that will set the ShowGroupTree property condionally. You would probably do this based on report name. In the Viewer load statement you could create logic something like this:
IF reportname1.rpt THEN ViewerName.ShowGroupTree = TRUE
else if reportname2.rpt then ViewerName.ShowGroupTree = FALSE
END IF
The only otherway I can think of is to create a separate form and viewer for each report and call those from your main form.
Both have drawbacks especially if you have a number of different reports.
Hope this is of some help.
Regards,
John W.