The feed that you put inside your cockatiels cage must be protected from being contaminated by droppings. A lot of the cockatiel feed containers sold at the pet store will have a cover that protects the feed from this. It is best to clip a feed container to the cage instead of putting it on the floor as it can be easily contaminated on the floor. Some breeders will always feed on the floor as this is the way cockatiels will feed in their natural habitat, breeders will always keep any eye on the food if it is on the ground and replace if it is noticed to be contaminated.
Do not put any perches directly above food or water containers, as you will be changing the food too often to rid of waste. Like many other birds, the cockatiel will pick out the seeds that they prefer and discard the others, this usually means they will make a huge mess digging out the seeds they want from the bottom of their feeding container.
Keep an eye on the food and replace if you see it get low, be aware that empty husks on the top of the seeds can give the illusion that the feed container is still full, you should blow these away so you can see how much real seed id left. All seed food should be changed everyday, natural branches can be left until destroyed.