NSTableView drop refuse or redirect

I have a question about best practices to handle drag&drop on table views. Often I want to support only drops in certain positions. For instance, lets say I only want to support insertions (NSTableViewDropAbove) and not drop on (NSTableViewDropOn). I can do two things when I get an NSTableViewDropOn in -tableView:validateDrop:proposedRow:proposedDropOperation:. One is to simply refuse the drop (return NSDragOperationNone), the other is to redirect the drop to an NSTableViewDropAbove using setDropRow:dropOperation:. The question is what the proper way is to handle such invalid drops?

thanks,
Christiaan