These questions come up periodically.

See http://www.thestrikepoint.com/cgi-bin/tsparc01/tsparc01.pl?read=11325 and http://www.thestrikepoint.com/cgi-bin/tsparc01/tsparc01.pl?read=2407 for discussion threads on cleaning.

See http://www.thestrikepoint.com/cgi-bin/tsparc01/tsparc01.pl?read=1387 and http://www.thestrikepoint.com/cgi-bin/tsparc01/tsparc01.pl?read=2447 for cases.

There are other threads too. You can always search the current messages and archives using the appropriate keywords.