I (almost) always wash the brush under hot running water before I apply the paste. Because, it's been in the bath environment and I think it's just an additional hygiene step I've always followed to clean the brush before brushing my teeth with it!
Wet toothbrush firstly. Gets rid of anything on it since last brushing. Just a guess, but I think that started when tooth cleanser came as a powder.
Does anyone else remember Ipana Tooth Cleaning Powder? It came in an ovally-shaped green & white tin!
So I suppose that practice just continued when 'paste' became the norm!
If you put paste on firstly, then wet all, how's the water going to get onto brush under where paste is sitting?
No, I’ve ALWAYS wet my brush 1st, but I turn the tap off again, until I need to rinse my mouth out & clean the toothbrush. And YES to donjo. I remember my mother, who’s now 93 & 1/2, using a white tooth powder, but I think it was in a small red/white tin. Well fancy someone else remembering about tooth powder. I was pleased to have read about that! I’m in my late 60’s now.