Are deadlines important?

You bet! I’ve heard many editors talk about authors missing deadlines and how it can throw off the whole process. Of course, there are times when there isn’t anyway a deadline will be made. As soon as you know, let your editor know. If you are asked to set a deadline, be reasonable in your estimation. Build in time for life to happen if possible.
You say you aren’t published so deadlines aren’t important. Start preparing yourself for when you will be published. Set a deadline for yourself and see if you can make it. It’s good to train yourself in keeping deadlines. Writers are professional and we must act as professionally as possible and treat our writing as a business.