When should you start mixing?
Mixing is all about having a vision on how things should sound. This vision should guide you right from the start of the production process so that each decision you make (whether its choosing a specific guitar, effect, microphone etc) will bring you one step closer to your desired sound.
Whenever I'm working on a new project I start off with creating a rough mix of the initial arrangement, balancing the different instruments, using EQ, Compression and adding different effects to the instruments that I already have. Working in this way ensures that every instrument has its own space in the mix as well as creating space for new instruments.