Most optos are reasonably fast - compared to a relay that is. So, if a switch is used to operate the opto - bounce is still an issue! Regardless of whether an input is a switch or some other means, it is always wise to implement some form of debounce or input filtering on a microprocessor - when you read a port pin there is only a very small window of time that the input is looked at - this could be a fast transient so it pays to look a few times just to be sure.