Hello Ashley! I'm gonna answer your question with only my opinion of course. There will always be other factors that come into play when you have to decide whether or not it would be okay to leave the dog outside all the time.
What kind of dog is it? Is it hot where you live all the time? Is it cold where you live all the time? Are your neighbors close enough to you that it would pose a disturbance if the dog were to bark or whine? Does the dog have a proper set-up to be outside? Does he/she have water and food available at all times? Is the dog aggressive, so that if someone would come into the back yard he would attack them?
In my personal opinion I believe it is cruel to keep a dog outside all the time. I think people get a dog for companionship, and to keep them outside all the time would be neglectful. I have 2 dogs. A golden retriever, and a brand new lab puppy. I also have kids, and so to keep the them outside all the time, away from the kids, would be torture to both!
When it comes down to whether or not you decide to keep your dog outside all the time, I would just make sure that you take into consideration all the factors that I listed above to make sure the dog is most comfortable. Good Luck!