There is no difference between US ASCII and UTF-8, so there isn't any need to reconvert it.
But here a little hint, if you have trouble with special-chars while recoding.
Add //TRANSLIT after the source-charset-Parameter.
Example:
iconv -f ISO-8859-1//TRANSLIT -t UTF-8 filename.sql > utf8-filename.sql
This helps me with strange types of quotes, which are always breaking the character set reencode process.