Wash my jars every single time. Leaving old bits or soap residue can mess with the good bacteria you want. I had a whole batch of sauerkraut go moldy once from a dirty jar. It's just not worth the risk of wasting all that food and time. A quick rinse with hot water takes two seconds.
Watched my friend Sarah lose a big batch of kimchi because she reused a jar without washing it. She thought a quick rinse was enough, but old chili paste stuck to the sides. The whole thing turned pink and fuzzy after a week. She was SO upset, all those veggies gone. Now she scrubs everything with hot water and vinegar. It's a lesson she won't forget.
Forget the jar, check the lid grooves too. Gunk hides in those threads and ruins ferments just as fast. Found that out after losing my own pickles last month lol.