$scriptdir will point to the directory containing the currently executing script, so it will point to wherever someone installs your script. If you use it from the command-line it defaults to the same location as $mircdir, but that's irrelevant.
If you use $scriptdir in your scripts when referring to resources the script will use but will not change then you'll be fine, for everything that will change (ie. users settings etc.), they should be put in a subdirectory of $mircdir for the reasons I mentioned previously.
