Hi there...what a great question!
Some personal background: I started with canine massage professionally in 2000. I also have had dogs my whole life (42 years!). I was bit in the face as a kid when I was unsupervised and in a dog's space - this was a friend's Husky. I yanked my face back so fast that I ended up only with scratches but went to the hospital anyway just in case. That was a major lesson for me and if the Husky really wanted to have done some damage he/she could have - I was very lucky. I have never been bit since or with canine massage. I have been mouthed, however, while working with shelter dogs who needed to learn boundaries (apparently I did, too as a kid, lol).
One of the best things you can do is volunteer at rescues to get your hands on as many dogs with varying body types and personalities as possible. Sometimes these organizations have informal canine behavior classes. Get comfortable around them. Just because we love animals doesn't guarantee that they will love us. Learning as much as you can about canine and animal behavior will help you greatly with your concerns about being bit - be safe.
I also work with horses. As a practitioner my safety (and theirs) is very important. Sometimes I am asked to work with aggressive acting animals. These are the risks that I take and they are very real. I have a permanent shoulder injury from a misbehaved horse, and I've been stepped on - thankfully never kicked.
One thing that I believe is a great help is to know energy work along with massage therapy (Reiki, Polarity, Therapeutic Touch, etc). If the animal is very sore, they can misbehave out of shear pain. So I go in and do some energy work first, then usually I can get my hands on them with massage.
One of the very real risks of this profession is what actually draws me to it. I am fascinated by animal (and human) behavior. Massage has profound impacts on behavior. I see a lot of the behavioral issues as just part of the challenge of working with animals.
Being aware of my intentions and how I am feeling the day I am scheduled to work on an animal is key to my success. I leave all my baggage at the door and can happily pick it back up when I'm done. Animals are very sensitive to the energy we carry and our thoughts. If I am distracted, so are they. If I am afraid they know it and will act out.
The main difference between working with humans and animals is that communication is accomplished through highly-tuned observation skills. Watching subtle ear or eye movements for example. They can't verbally say what hurts and some show levels of pain differently, so observing their reactions to the massage is important. Learning biomechanics of the canine will help hone observation skills, as well as just spending time around them. Watch dogs interact with other dogs - watch their body language, posturing.
So, my advice is to volunteer at a dog shelter - pick up a canine massage book, practice some strokes and really connect with them, observe behavior/body language - before taking the canine massage course! Enjoy!