My take on the Discworld witches is that each one has a fairly defined character which in turn defines them in both their daily life avoiding the use of magic, and in those times when they have to use their magic.
Granny weatherwax avoids using magic by using headology, but when she uses magic it is often borrowing (extreme headology)
Magrat avoids using magic by using natural remedies, and when forced to use magic she uses nature ('growing' cellar doors and accidental pumpkins)
Tiffany's solution to the day to day stuff is to think logically and analytically, but her magical side comes through in her 'third thoughts' because when she uses those she can work out how to do thing that make granny Weatherwax sit back and take notice.
Nanny Ogg's approach is pragmatic and bawdy, and what she knows of human (sexual) nature enables her to avoid magic - unless she really needs a good love potion. knowing human nature so well also means she is the one person who realises when Granny is about to start cackling, so she looks after her old friend.
Miss Tick is a natural teacher so her magical side is her ability to 'detect' a growing witch
I can't go further without re-reading all the witch books and analysing them to death and that would just take the fun out of it all.
but them's me thoughts
Sue