How does a voltmeter work?
Voltmeters measure the voltage they see after loading it with their "input resistance".  For meters with electronic circuitry, this is usually in the 10 M&Omega; range.  Therefore, the Ro impedance of the source voltage and the voltmeter input impedance form a voltage divider, and the voltmeter tells you the voltage out of that divider.

As long as the source voltage impedance is substantially less than the voltmeter's input impedance, the voltmeter will read the voltage accurately.  This is why higher input resistance is better for voltmeters.  In practise, most of the time loading something with 10 M&Omega; won't change its voltage to the point you care.  Still, you have to be aware of the limitations of your tools to know when they won't give you accurate readings.