/.swf will not block so much, with $ at the end or without: The . will matach any character, but the / will only match /. So this line will only block URLs that contain strings like '/aswf' or '/Xswf', which aren't very widespread. ;-)
If you want to block flash, the best thing will be to look for URLs with 'swf' at the end of the line. The corresponding line would be:
What you have seen is probably the hint to use the line
or similar to block flash. This means: A /, then any character (.), zero or multiple times (*), then 'swf' at the end of the line ($). As slashes tend to occur in about any URL (at least twice at the end of 'http://'), this works, too, but it is unnecessarily complicated. I don't know why many people seem to use the slash at the beginning—probably because we're used to using slashes with regular expressions (in many programming languages, regular expressions are encircled with slashes).
If you want to know more about ad blocking with regular expressions, you might want to read this: http://forums.omnigroup.com/showthread.php?t=2036