You've heard the old expression; "Never assume because it makes an ass out of you and me".
I'd like to suggest an alternative that may be a little more useful in an instructional design world; "Never assume unless you assume the worst case scenario".
I design to that worst case scenario all the time. I like to assume that the person delivering my training has no knowledge of the subject and little to no training experience. How many times have you received a PowerPoint file with no leader's notes or delivery instructions? These types of files are typically designed by people who expected that they themselves would be delivering the material. That or they simply do not care to put the extra effort in.
I think a general business rule should be to leave instructions to your replacement. This is especially applicable to leader’s notes in training. While you might be off on the next project, someone entirely different may be required to deliver your course. It just makes sense to provide them the context needed to do a great job. Not only is it professional, it's a testament to your instructional design capabilities.