You could set this globally in the MainWindows constuctor.
Something like this:
RenderOptions.ClearTypeHintProperty.OverrideMetadata(typeof(FrameworkElement), new FrameworkPropertyMetadata { DefaultValue = ClearTypeHint.Enabled });
TextOptions.TextFormattingModeProperty.OverrideMetadata(typeof(FrameworkElement), new FrameworkPropertyMetadata { DefaultValue = TextFormattingMode.Display });