The concern with using a surge tank is the coolant is not circulated through the tank on a conventional setup. If yours is like mine, the surge tank is pressurized and the coolant circulates through it so the mix there should match the mix in the entire cooling system. In my case, I am OK using the surge tank (actually called a DeGas bottle on this kind of system).
If you can get a clean sample from the petcock that is the best, as stated earlier. Using the petcock be careful not to contaminate the sample with grease, oil, road dirt and such that has built up on the underside of the truck around the bottom of the radiator.
Dave / Believer45