It would be good practice to first search the stylesheet to see if one of the selectors you are adding is already listed; if so, keeping related rules together will make it easier for you to maintain the stylesheet.
Generic rules like
a img {what: ever;}
should go before specific rules like
#categories a img {what: ever;}
so the specific ones are not overridden by the generic case.
Some people put all of their new rules at the end of the stylesheet.
In the end, whatever functions correctly and is easy for you to maintain is "right"... unless you are working in a corporate or other environment where somebody else may have to work on the code. In that case, following defined standards becomes very important.