A dirty cage is an invitation to illness.
Your cockatiel's droppings and the food you feed that ends up on the floor of the cage will harbor bacteria. The cage floor should contain a metal grate to prevent the bird from scavenging through his feces for a treat he dropped.
Corn cob is not recommended for covering the bottom of the cage. Cockatiels have been known to get through that metal grate and eat the corn cob, which can impact their digestive system and cause them to literally starve to death. Corn cob bedding is also an excellent breeding ground for aspergillus, a fungus which, when stirred up by wing flapping, can be inhaled. This fungus will then begin to grow inside the air sacs and lungs of birds, and is almost impossible to treat unless detected very early.
Change the bottom of your cockatiel's cage every few days (or more often if possible). Wash the cage bottom weekly, and every month, disinfect the entire cage thoroughly. Diluted chlorine bleach is one of the best and least expensive disinfectants available, but you must rinse everything (perches, grates, toys) thoroughly before putting your cockatiel back into its cage!