You don't need to, no. The only character from Equal Rites who reappears in Wryd Sisters is Granny Weatherwax -- and Granny is totally self-explanatory. As is the setting. (Besides, the action of Wyrd Sisters is set quite a bit later than Equal Rites.)

I think most of them can stand alone, even if the characters reappear.

All are standalone aside from the first two. If you do end up reading Equal Rites first, it will only get better from there...

Ooh, interesting. I've never listened to the play.