Actually using clean water on most depowered electrical components is safe to do.
I use to think the same thing and always used the toothbrush method until my professor in college told me this. At first I was in total disbelief, but a little research and a trial run proved me wrong. When I run them in the dishwasher, I just use the rinse cycle with the heat turned off, then remove them right away so they can dry.
I still do use the toothbrush method for small clean-ups like a blown cap that leaked and stuff. The problem I’ve found with using a toothbrush method is that it won’t reach very well if at all under a lot of components on boards.