Often, people think a long haired dog should be shaved during warm weather. While on one level that may seem logical, but in reality, it is not wise to do so with a Sheltie. The Sheltie coat not only acts as protection against cold in the winter, it serves the same purpose in the summer, protecting the Sheltie from the heat.
The Sheltie coat also protects against parasites such as fleas, ticks, mosquitoes. That's not to say a Sheltie never encounters these parasites, but that thick coat does slow those critters down. Another advantage to the Sheltie coat is that it repels dirt. Get rid of that coat and you're inviting dirt to attack your Sheltie and cling to its skin.
Also, the Sheltie coat helps to prevent that "doggie odor" that so many breeds have. Shelties don't typically have a "doggie odor," partially because of the coat. Take that coat off and the Sheltie may begin to smell like a dog.
While shaving may be necessary for medical purposes, such as in preparation for surgery, shaving just for convenient coat care should be avoided. Shaving too close can cause medical conditions from something as common as razor burn (which hurts like the dickens) to more serious damage. Some damage can be permanent, such as damage to the hair follicles. The fur may grow back in patches, leaving the Sheltie bald in various places all over its body, or the fur may not grow back at all. If the care of coat is inconvenient or too difficult, reconsider if the Sheltie is the right breed for you.
That luxurious coat is what makes a Sheltie a Sheltie. They know they are beautiful with that gorgeous coat. Most Shelties have actually acted embarrassed after being shaved. So, for the health, beauty, and pride of your Sheltie, please don't shave.