Actually OpenSprinkler already has two built-in fuses: one connected in series with the 24VAC common wire protecting solenoid shorting, and one protecting the +5V line. Also, there are 10 bidirectional TVS (or MOV in DIY kits) to protect the circuit from transient high voltages.
I did think a lot about adding opto-isolators, and decided not to adopt them for the moment, because the opto-isolator is bulky, expensive, and in the event of shorting it has the same risk of leaving the solenoid open. It does, however, isolate the microcontroller so the microcontroller will not be damaged.