When you go to speak somewhere and you are offered a microphone, it's generally a good idea to use it. Keep in mind that the people running the event will be familiar with the venue and will know very well wether your voice needs to be amplified to be understood by the audience members in the back of the room.
You may be reluctant to use a microphone simply because you are not familiar with its correct use. It should be worth this small discomfort for the benefit of your audience, though.