Is it a good idea to use an opto isolator between the Arduino output to a midi out connector which will be sent to a keyboard midi in?
I have seen plenty of schematics for an isolator between the midi input to the Arduino but none for the other way around.
I ask as I am afraid that the output from the Arduino to the keyboard midi in could perhaps send a too high a voltage signal to the keyboard or am I worrying needlessly?
Right. There's supposed to be an opto-isolator on the inputs, but not the outputs. If there is an opto-isolator on every input then every connection is optically isolated. [u]Here is some reference information[/u].
If everything is 5V, over-voltage shouldn't be a problem. I'm not sure why there's an opto-isolator... Maybe for ground isolation (to prevent ground loops). An opto-isolator can protect the rest of the circuitry but it's not really over-voltage protection because you can burn-out the LED inside the opto-isolator.