If you add the code to the plugin stylesheet and ever update the plugin, the code you added will be overwritten. That's why you should put any extra code in the THEME stylesheet.
And I always add comments above and below the new code, because as time goes by, you will forget what code does what - not required, but it has saved my butt more than once.
/* added 1 Feb 2013 to allow display format and category dropdown lists on ai1ec to work correctly */
code
.
.
.
/* end of formatting code */