The skin can fold up into the matt quite far. You cut the hair and end up getting the skin. Plus you have to worry about moles, skin tags and etc. You may think you know where the skin is but you really dont.

When I use snap on combs I just make sure they are matt free plus after 15 years I have the feel that I know at what point they can pop off. But in all the years I groomed I have seen many many times a groomer grooming along with a snap on and boom it pops off and they end up taking a chunk out of a coat. Not a big deal if it is your own dog. But customers get upset if you take a chunk out of their Bichon's fluffy hair cut.

IMO Unless it is summer a 7 on a Shih Tzu is WAY too short. You tend to see skin on a thin coated dog. A #7 cuts at 1/8". A #5 or #4 is better IMO. Make sure you get FULL TOOTH blades. Skips leave too much chance of a noob nicking a dog.