If I were thinking about a bob design for the uc300 - 5LPT, would it be recommended to use optoisolators for the step and direction signals to stepper drives or could direct connection from the outputs be done at 5v?
It depends on if the stepper drive has the optoisolators on the inputs. Most stepper drives have that, so then the outputs of the UC300ETH-5LPT can be connected directly to drive the optocouplers.