Using Subversion for source control and need to know which files in particular need to be controlled. The obvious ones to me are the project file (.ism) and the script files (.rul). Any others?
That sounds correct, storing the .ism project file (saved in XML format) and the .rul files. Depending on your project type, any auxiliary external .vbs, .sql, or similar files that you've been editing in the InstallShield environment should go there, too.
