This question is one of my favorites. I have gotten this question from people with cute new puppies, dogs they have had for years, and rescue dogs, basically everyone will ask this questions. The answer is it depends. Wait... did a dog trainer really just say you don't always need a dog trainer? oh yes I did. A lot of it depends on what you want from the dog and, in all honesty, a lot of it depends on your budget. 
If you don't care what your dog does and if it listens to you then no need for any training. If you care but can't afford a trainer then you learn as you go, and aren't we all glad for youtube and google. However if you can afford a trainer and you want a well behaved dog I recommend hiring one. 
But wait a moment, even if you have trained your own dog before you should hire a trainer? I recommend it. why? Because not all dogs are the same. Individual adjustments to training are a good idea. The trainer also provides a second pair of eyes, that shouldn't be biased, that can help you catch things you've missed. This is why I don't sell packages of 10 lessons, but rather set things up with you on your dog's needs. There may be some dogs I only need to work with once or twice- basically doing some finishing touches. There may be some dogs that I need to meet with 10 times and help you get it under control, and of course there is the happy middle ground. 
To go with this, with my dog Stinker, the beautiful malamute on the banner, I have had several of my trainer peers work with her with me. What? A professional dog trainer enlisting other professional dog trainers? yes. They provided me with some insights that I was missing, because literally, I was too close to the situation. 
However again I want to note, in some cases, if you can't afford a dog trainer and/or you really want to do it yourself with the aid of online resources your dog will most likely turn out just fine. But I will always recommend considering getting a dog trainer at least to give you some insights you might be missing. 
Now if your dog is having aggression issues you need a trainer. if you dog will not respond to your commands you need a trainer. If you want to do a specialized certification you need a trainer (they have to sign off on it). If you dog is having other behavioral issues, for example keeps having accidents in the house, you need a trainer. I am being blunt. IF there is some issue you cannot get resolved, don't give away the dog, get a trainer. 
One of the most ridiculous stories I have ever heard was when a family went camping with their 12 month old lab. They went to go on several hour activity and left the lab in the trailer unkenneled. Now this is also the first time the dog had ever been in the trailer. Anyone want to guess what happened? Yep you got it. The trailer was destroyed. They decided the problem was their dog and they needed to get rid of the dog. No let me explain to you what I had to fight myself not to explain to them. They were idiots. A young lab is very chewy. Especially when they are nervous or stressed. Guess what leaving the dog unattended for hours in new environment=mega stress. Now if they had had the dog kennel trained and in its kennel there would have been no problem. Now did the mean to be idiots.No. They honestly didn't know it was their first and according to the parents only dog. They found the dog a new home. I applaud. and that is the end of their dog story. 
Now in my opinion what should have happened is they should have 1. been more observant and conscious of their dog. 2. IF they had been working with a trainer. any trainer worth their salt would have had them kennel train their puppy and would have walked them through appropriate times to kennel your dog. That camping experience would have been an appropriate time to kennel their dog. Even after the destruction tornado ransacked their camper rather than dog's bad they should have realized they screwed up and worked to train the dog so it was never a problem again. That is what the responsible dog owner does. They figure out what they need to do better and train the dog so there is no longer a problem. They do not get rid of a dog.  
Anways, now that rant is over. Lets do a quick recap. In some cases you don't have to have a trainer, but there is not a case I am aware of where having a trainer is going to hurt. Also in most cases it is extremely beneficial.