Citations can be a messy thing. They're actually simpler than most people think, but everyone likes to make them messy. Citing your own work, of course, does add a layer of complexity.
At its simplest, using someone else's idea means you need to cite them. There are two reasons for this. One is that you should give credit if your idea actually isn't yours. It's only right. Even if you put it in different words, it's still their idea. Second is that you should give credit to actually lend some credence to what you're saying. That is the part most people don't realize. Often we're taught that we need to be creative and think of things completely on our own, but since when are either you or me the world's expert on any given topic? Better to apply what the experts are saying than to just make something up yourself. It's not weak to use someone else's idea, but it actually makes your argument better.
That said, related to self-citation, there are two principles at play. One is copyright and the idea of giving someone credit for an idea you're using. Obviously if you write something, you own the copyright to what you create, and you can copy what you wrote verbatim or put in different words as much as you like, since it's your copyright. Beyond copyright, however, the idea of self-plagiarism comes into play, as a particular ethical issue of higher education that is not really applicable elsewhere. Generally speaking, professors don't like you using something you wrote for another class in their class, without permission. Sometimes this varies based on the professor, and other times it is an institutional policy. Where I currently teach, they don't have a policy against this, because as part of the competency based model, it's not likely that something a student writes for one class will work in another class without major revisions. If you have published something, it's yours, so do what you want with it. If you think citing yourself will lend additional credence to what you say since it's been published, then use it to make your presentation stronger.
Many schools use a service like TurnItIn, however, to check if something a student submits was submitted to another class or found on the Internet somewhere. So it all comes down to execution, where the rubber hits the road. If you can copy something you wrote elsewhere but the computer dings you for it, you'll have to deal with it and explain what you did, even if it was perfectly okay to do so. If you make it clear up front what is going on and cite everything, then it doesn't look as much like you're trying to hide something.
