Just for reference, in America there is no certification for service dogs. And technically there's no required "certification" for therapy dogs either. Therapy dogs must be registered however, and they get that registry through the therapy groups that sponsor them to go into special settings. Yes, you can go the extra mile and get the certification from therapy dogs international, but a lot of the programs you can volunteer with have a trainer that's qualified to give the CGC which must be passed and then put you in the registry.
With service dogs it is fully recommended that you do your CGC for several reasons, one if you get taken to court you have documentation saying your dog is above average in terms of training, two it's a good way to know if you're ready for the "real world". Most service dog handlers prefer to take it one step further and go on with a PAT, I know of one organization that holds them in various states at various times. You can also do it while being monitored with an expert trainer, etc. Just get it on video.
Keep in mind, state laws are different for service dogs in TRAINING. The ADA only covers fully trained service dogs. For instance, Illinois says that I can take my service dog in training to public areas and be treated as if my dog is fully trained. Texas, though, only allows school/organization/group service dogs in training in public. Some states don't allow in form of in training dog in. If you're in one of those states just try to find dog friendly places, petsmart, cafe's, or places you've gotten permission from the owner.
Be sure to keep an in training badge on your dog's vest if he's not fully trained! People are more forgiving and if your dog slips up they don't expect the fully trained dogs to act like that.
It's up to girlscout/boyscout honor really that a handler will be responsible and only start to take their service dog to "human only" locations once the dog exceeds normal standards and not til after you've passed the CGC. Remember, the dog must be task-trained to be a service dog and the task(s) must be relevant to your legally defined disability.
Just remember, the impression you leave with store owners can either benefit other responsible handlers that follow or do a great injustice to them despite the laws. Some shop owners refuse service dogs after a bad experience with irresponsible handlers or fakes and even when threatened to have the police called or the ADA brought into play in court they still give responsible handlers problems.
All things to consider.
