How to intelligently & safely convert a Double to String?

If you could live with strings like 00, 05, 10, 15, 20 ... etc., you could simply use

(rating * 10).ToString("00")

If not, use InvariantCulture as argument to ToString in order to force the use of a decimal point (".") in all countries (in Germany the default would be "," for example):

rating.ToString(CultureInfo.InvariantCulture).Replace(".","");

Decimal delimiter depends on current culture preferences:

d.Replace(
    System.Globalization.CultureInfo.CurrentCulture.NumberFormat.NumberDecimalSeparator,
    String.Empty)

will replace '.' or ',' with ""