ATmega328PB and ADC

I couldn't understand if the first conversion after ADC is enabled (ADEN bit) must be discarded or not.
The datasheet suggests to discard the first conversion result after changing the reference signal, but nothing after enabling ADC or changing input signal (ADMUX).
The only note is about the conversion time that is longer for the first conversion after ADC is enabled.
I'm asking this question because I'm noting a different behaviour if I start conversion and enable ADC at the same time for each conversion against leaving ADC enabled and starting only the conversion (ADSC bit).