Keeping the food bowl full at all times is known as "free feeding." It relies on the idea that your dog will know when to eat and when to stop -- and it gives her the freedom to make her own choice regarding eating times. Whether that's a good thing or not depends on a number of factors, including your pet's weight and how many dogs you have.
Pros of Free Feeding
Possibly the biggest advantage of free feeding is that it's easy for you -- simply fill the bowl in the morning and then forget about it until the next day or until the bowl is empty again. Also, some dogs prefer to graze rather than eat large amounts of food at once. For them, having free access to food at all times might be better -- they can just eat a bit here and a bit there, as they feel like it. According to the ASPCA, free feeding is a good option for nursing dogs, as they can eat anytime they get a chance to step away from the puppies.
Cons of Free Freeding
Not all dogs are great at self-control and you might end up with a dog who eats more than he needs just because the food is there. If all pets were great at self-control, there would be no overweight animals in the US. According to the Association for Pet Obesity Prevention, over 53 percent of dogs in the country are overweight, so the system is obviously not flawless. According to WebMD, free feeding can make it hard to housebreak a puppy, as dogs usually have to go about 30 minutes after a meal. If you leave food out at all times, you can't possibly guess when it's time for a bathroom break.
Free feeding might not be a good option if you have more than one dog. Some dogs are extremely food-possessive and having food available at all times can lead to fights over who actually owns the bowl. Never mind that you might be willing to leave more than one bowl out -- the possessive dog might decide that he needs to take over all the bowls.
Free feeding only works if you feed your dog dry kibble. Wet food -- or dry food mixed with human food or treats -- can attract insects or rodents. It can also go rancid, especially if the weather is hot -- leading to tummy trouble and a very sick doggie.