When should you start mixing? From the very first track?
We take a lot of our current operating procedures from the traditional days of recording. Before DAWs, it was difficult or near-impossible to store and recall a mix. So during recording you would use a relatively crude 'monitor mix' and save mixing for real until after every note of music had been laid down.
(The alternative was to book a studio on 'lockout' so that the settings of the mixer could be kept intact overnight.)
Also, in the modern world of music, it is common that mixing will be carried out by a specialist mix engineer, so again there is a separation between recording and mixing.
But when you think about it, as you add new musical lines to a recording, shouldn't you be adding them in the best possible way, that contributes most to the quality of the production?
Why would you ever add a new musical line in a half-hearted way and not consider fully how it will sound in the finished piece of work?
It makes sense therefore when you add, say, a rhythm guitar part to optimize the sound from the guitar, amp and microphone. And then in your DAW's mixer set the EQ, pan and level that sounds exactly right next to your previously recorded tracks.
If you do this all the way from the beginning of a new session to the end, there need be no mixing process because you have built up the perfect mix for your song as you have gone along.
Of course, there will always be minor tweaking. But as a way of working, mixing as you go is a tremendously valuable technique.