It depends on where the dents are and how deep.
Dings of the case mouth can be fixed during the resizing process. If the expander button will not fit, use some other tool to get it "round" and resize as normal. If the mouth is creased to much, it will collapse on resizing.
Small dents in the shoulder will fire form at the next firing. These usually occur during resizing when using too much lubricant. The lubricant gets on the shoulder and causes the dents. If a should dent extends past the transition to the body, I scrap the case.
Body dents would form out during firing but are probably have the highest rate of discard than any other. Generally, a small ding, will ignor but usually the dent is more substantial and just do not bother trying to rescue the case. Usually, I find larger body dings come from other sources other than the rifle (size 13s come to mind) but i suppose the rifle could make some dents.
Some semi-auto rifles are hard on rims and tend to bend them up or tear them. The dents make inserting the case in a shell holder difficult. A little file work can improve the situation but it can get to be alot of work if your rifle is real hard on rims.
Hope this helps.