I would also add that unless you know your EPUB/MOBI formats are impeccable you may want to eyeball them before you do a mass delete of other formats for that book. There are plenty of very poor quality EPUB conversions floating around out there for instance.
If you don't have time to eyeball them all now I would recommend archiving all the other formats in case you need them someday - they don't have to be in Calibre, just accessible in case you find you need to do your own re-conversion if your EPUB you kept is irrecoverably crap.
