All values that the string representation can be parsed as double, because SharePoint converts the input value to a string:
case SPFieldType.Number:
case SPFieldType.Currency:
str1 = Convert.ToString(value, (IFormatProvider) CultureInfo.InvariantCulture);
break;
(from SPListItem.SetValue (...))
You should be okay with string, int, doubleetc.
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