Should I declare converters in App.xaml or as a per-file resource?

Well, I just don't declare them in xaml at all. Instead, I additionally derive a converter of mine from MarkupExtension. Like this:

public class MyValueConverter : MarkupExtension, IValueConverter
{
    private static MyValueConverter _converter = null;
    public override object ProvideValue(IServiceProvider serviceProvider)
    {
        if (_converter == null) _converter = new MyValueConverter();    
        return _converter;
    }

    public object Convert
     (object value, Type targetType, object parameter, CultureInfo culture) { }
    public object ConvertBack
     (object value, Type targetType, object parameter, CultureInfo culture) { }
}

This allows me to use my converter anywhere, like this:

Source="{Binding myValue, Converter={converters:MyValueConverter}}"

where converters is the namespace in which I have declared my converter.

Learned this trick from an old stackoverflow thread only.