As I understand, Ethereum smart contracts can store data internally. I am wondering - is it cheaper for a smart contract to keep all of its data forever if it is keeping the data for any length of time, or does deleting unnecessary data from the contract after it has been used better (even though you have to pay for the operations)?

For example, if I have a contract that keeps a record of everyone that triggered it in the last day, does it make sense for me to remove that data the following day when it is no longer needed, or since the storage was already paid for, there is no point in deleting the old data?