No, Grosh will normally do this automatically. It is always good to ensure the category is correctly set when adding new types of groceries, and correct Grosh if the app makes a mistake. Correct categorisation will give you a correct grouping of items on the shopping list. This helps you clear all dairy products whilst you are in that area of the shop, then move on to the next area to get the grocery items related to that category. This will make you a more effective grocery shopper!
Note: Categorisation of items is personal and therefore nothing to do with the selected list. This means that if you have several lists and add the same item to them, the category you picked on one list will be reused on the other.
This can certainly have benefits if you have several lists, but there's downside: A side effect of this is that your spouse may see a different categorisation than you for the same list - as your spouse will have his or her own categorisation defined.