If you apply the 'assume good will' rule to everyday life it saves you a lot of unhappiness and also ensures that you behave in a moral way. If somebody does something that has negative repercussions for you, you're likely to get upset for two reasons: firstly, because something bad has been done to you and, secondly, because this implies that whoever did it doesn't really care about you. Now, it may just be me, but I think the one that's likely to cause the most stress is the fact that somebody probably doesn't care that much about you. So, if you 'assume good will' when somebody choses to do something bad to you, you won't feel as bad, because you'll be assuming that they didn't have malicious intentions, and to be honest, the majority of times when people upset one another, it is a misunderstanding and things would not escalate so badly if the other person had assumed good will.
Now, my final point, adopting this maxim would make you a better person, and here's why: Think of something bad you've done, the worst thing. When you have done things to people which are 'immoral' you haven't ever done it just to 'be bad' or 'for the evulz' have you? Every immoral thing you've done, will probably have stemmed from the fact that you didn't assume good will when somebody inadvertently wronged you. You immoral actions were most likely revenge or something similar. Assuming good will, will mean that you avoid this, conflicts won't escalate as often and you can talk it over with the other person involved in a relaxed, peaceful atmosphere.