Charging the battery at any point should not affect the life of the product. The headphones and charging case will be able to complete a certain number of charging cycles. This means that it can go from 0% to 100% a certain number of times. This does not mean that when you charge from 70% to 100% that 1 charging cycle will be completed. Therefore, you would have to do a full 100% worth of charging to complete the cycle.
I hope this makes sense! If you have any questions, do let me know!