The ignore option operates on a file/folder level in your working copy. Right click on an item in the Flex Navigator pane, go down to the Team menu and then select Add to svn:ignore.
With regards to why something should be excluded - mainly because there is no point. When you commit a source revision, it should contain everything needed to create the output binaries (source files, resources, compiler/project settings etc). The only thing you gain by putting your output folders under version control is to have more stuff bloating up your repository. Even more annoyingly, you will end up with a dirty working copy that gets committed where nothing has changed except that someone rebuilt the project - this clutters your history and forces other team members to do an update where non was needed. This is not specific advice for Flex, this is pretty much given for any development environment under version control. Regards, Nik
