Hey @keithyoung, do you mean dried bits? In my experience, a little fresh leftover mud is fine, but any dried gunk will definitely mess things up. I just wipe my bucket out well between mixes.
Honestly that tip is way too careful. A few dried specks never hurt anyone's mix in my whole time doing this. Tbh the new mud just breaks down the old bits and it all blends together fine. If you wipe it out too much you're just wasting time and getting your hands dirty for no reason. Ngl I've left way worse in my bucket and the next batch still came out perfect.