Assumptions are made whether we want it or not. The only things that aren’t made assumptions about are the things that do not catch our attention. So, it means it’s all the time.
And you can’t choose not to assume. That cannot happen. Not in the true sense. What you can do is trying to be a little bit kind and fair when making them. The reason is that in any case, you can turn out to be wrong.
Further, you should definitely try to be a good communicator about it. When people say “don’t assume,” what they usually mean is sharing them with the people involved so that there’s nothing left in the fog. Communicating your assumptions is not only minimizing the potential harm, but also a chance to revive/improve the relationship.