Over the entire voltage-temperature envelope of the 328P processor, with a goal of 38400 baud, the error is -6.73% to 3.92%. Well within range for serial communications to work.
Assuming the application does not require the full voltage range (1.8 to 5.5), the error over the temperature range drops to -3.60% to 2.66%. Low enough for serial communications to be considered reliable.