Optional referenced-type paremeter : Best Practice

I know in C#, by default, objects are passed by reference to a method then the changes reflect on the object outside the method. I have a function which sometimes I just need its return value and sometimes both return value and the input object are needed outside the function.

Then I wrote it as the following:

public static List<RuleSet> Load(XmlDocument dom= null)
{
   if (dom == null)
   {
       dom = new XmlDocument();
   }
   string path = ...
   dom.Load(path);
   List<RuleSet> ruleSets = new List<RuleSet>();
   // load the dom with file and read rules
   return ruleSets;
}

Sometimes I need the dom outside of the function and I call it Load(dom) and when I just need the returned value I call it as Load().

However I know if the parameter was declared with ref I couldn't set a default value for it and there must be a reason for that, While now I did it with no problem.

Then I suspect my approach to the problem (my function above) is correct or not, or could it be implemented in better way?