Is it bad to kill tasks in android, by using things such as holding down the home key and selectign "close all"?
Short answer is: "That's Ok for device, don't worry"

Long answer is: killing task is the only way to speed up your system. When you kill an app by task killer, you close the app itself and all services or receivers bounded to it. That action would be harmful for the application, if it's downloading some data from the internet (some content update for example). Or application will stop sending notifications about new events. Almost all core apps will raise again in a few minutes. That's Ok for device, don't worry.