If I'm working with wools...I knit along til I have only about 6" left...and then tie in the new skein using the felted join.
But, some yarns don't take to the felted join as nicely. NORO does great!
I don't mind tying in at the side edge...because sometimes a middle of the row tie in (with some yarns) is not as invisible as I'd like.
My tie-in methods vary from yarn to yarn, pattern to pattern.
The most important determining factor: an invisible transition, even if that means a little yarn is wasted for a side edge join.
Sorry I couldn't give you more concrete info!