dir = "C:/Users/steinr/ImageJ/_images/";
dir = "/Users/wayne/Downloads/";
dir = "C:\\Users\\steinr\\ImageJ\\_images\\";
File.setDefaultDirectory(dir);
print("dir: |"+File.getDefaultDirectory+"|");
exit;

//The following does not work:
dir = "C:/Users/steinr/ImageJ/_images/";
call("ij.io.OpenDialog.setDefaultDirectory", dir);
print(call("ij.io.OpenDialog.getDefaultDirectory"));         //works
open("");            //fails (defaults to last used directory when opening files)
exit;
 
//The following works as expected:
dir = "C:\\Users\\steinr\\ImageJ\\_images\\";
call("ij.io.OpenDialog.setDefaultDirectory", dir);
print(call("ij.io.OpenDialog.getDefaultDirectory"));         //works
open("");            //works
exit;
------------------------------
 
Demo macro for default saving dir:
------------------------------
//All the following works as expected:
dir = "C:\\Users\\steinr\\ImageJ\\_images\\";
call("ij.io.OpenDialog.setDefaultDirectory", dir);
saveAs("TIFF"); //works
 
dir = "C:\\Users\\steinr\\ImageJ\\_images\\";
call("ij.plugin.frame.Editor.setDefaultDirectory", dir);
saveAs("Text"); //works
 
dir = "C:/Users/steinr/ImageJ/_images/";
call("ij.io.OpenDialog.setDefaultDirectory", dir);
saveAs("TIFF"); //works
 
dir = "C:/Users/steinr/ImageJ/_images/";
call("ij.plugin.frame.Editor.setDefaultDirectory", dir);
saveAs("Text"); //works