For the voltage divider equation to hold, the output current must be made negligible relative to the current through the resistors.
IR > 10 Iout
I often see that, for the base biasing divider:
IR = 20 IB
I suppose the same conditions must be met also with the RE and RC but cannot find any guides verifying this.
Except when the Vc is almost equal to Vce and hits Vcc and the emitter resistor is momentarily off (almost off with the negligible reverse saturation current through the base-collector diode)from the circuit; RC and (RE+Vce/Ic) form a voltage divider (counting the RE and the transistor in series as one resistor and RC the other).
I know that RC must be as lower as possible than Rout to make the output behave more like a dead end in terms of current. But I don't know the practical rule of thumb for this if any.