Every now and then I'm tempted to buy another puck of Williams Mug. I wound up looking at a YouTube article on producing lather from Williams, and the guy advised never rinsing the soap out of a brush after use -- just leaving it soapy, and presumably letting it dry. Somehow I can't believe this is OK. In my way of thinking, it would be a mess, for starters, and maybe not so good for the brush in the end. So tell me, is this advice reasonable, or not. I've never heard or seen of it before. Does it contain any beneficial foundation from the present or past practices? I've always rinsed my brushes thoroughly after use. Comments?