Staying with relatives in China?

If you are in transit to a third country there is no need for invitation letter. You will need to register at local police station since you are not staying in a hotel. Its best to be accompanied by one of the Chinese speaking relatives or friends to handle the process

Thanks. Would having a letter be helpful though for the immigration officer and lower your chances of getting denied randomly?

You will not get denied. There is no reason for such a drastic measure. In reality, you can bring this invitation letter, but keep in mind to have a name and surname, an address and the phone number of your host in China because these details are required while filling out the blue form at 144 hour visa free transit counter. Of course, you should not forget to register yourself within 24 hours from your arrival in China.