Personally, I am thrilled when I encounter people who can sign. To me, every little bit helps. You don't need to be fluent for us to have a conversation. It is not rude to be learning to sign. As with any language there will be some early stage of learning and mistakes but if you don't practice you won't improve.
It is good to ask if the person signs before continuing in ASL rather than assuming every deaf person signs or wants to sign with you. Follow the deaf person's lead on how s/he wants to communicate. Depending on the environment, I may have already prepared a hand written note, other times I may point to what I want, other times I may use my voice.
