The FAST or SLOW slew can cause as much as 5% duty cycle variation depending on the frequency and the load.
The general recommendation is to use FAST slew rate for frequencies higher than MHz. At higher frequencies, FAST slew will show a more balanced duty cycle and also the faster edge rates can keep up with the faster data switching rates.
SLOW slew rate can be used at lower frequencies but as the design run faster, SLOW slew will cause more duty cycle variations. Also the slower edges are not fast enough to meet the frequency requirements.