Setting up a custom fee will increase your privacy. If someone knows your service fee, they can make Blockchain-analysis (check the time and amount of your transactions and figure out the destination account). It's easy to monitor the fee and see what is going on. For example, if you send 1 ETH with a fixed fee of 0.1% + 0.0001 ETH, the recipient would receive 0.9989 ETH on the other side.