Thats true the amp is self biasing but you still need to get the idle current correct for the EL84 output valves at the outset and thats what you are setting with the correct value of bias resistor.
(NB If you change a ECC83 valve that will regulate itself by default.)
Here's the analogy.
Before being able to drive your car down the street, you need to start it up and it needs to idle. Using an old fashioned Carborettor as an example, you need to get the air/petrol mixture correct and thats done by tweaking the mixture screw. If the mixture is set incorrect it will either run too rich or too lean. The car will probably still run, but either of those two settings would not be conducive for optimum performance.
The same things goes for the EL84's Output Valves, you need to tweak the ("mixture screw"), in the case of the amp its the the bias resistor. This sets the quiescent current or idle current going through the valves. In the case of the EL84's you are looking for 12Watts on the Plates. To work this out you need to take voltage and current measurements and then by using Ohms Law work out the Plate discipation (Power (watts)= Voltage x Current.)
Taking my Made In Korea unit as the example I worked out with the "factory fit" 120 Ohm Bias resistor installed, there was actually around "15W" on the Plates in the 15 watt setting and around "8W" on the Plates in the 7 watts setting. (Ideally it should be 12 Watts on both) Taking the car analogy one is too rich, one is too lean.
To my ears, once I'd got things right, the amp sounded a lot fuller and fatter and all that horrible fuzzy fizz at high gain settings had gone.
With regards mixing and matching: you are probably referring to people mixing and matching the ECC83 Pre-amp Valves.
Not so the EL84 output valves though, they ideally need to be a matched pair.
However, I wouldn't worry too much about it, if you are happy with the sound of your amp leave well alone. The only issue would be, if the Plate discipation is too high (>12Watts), then the EL84 valves will burn out a lot quicker.