We had this Irish SEO company (so I need to be careful here), "liberate" a client in the promise of some top class ranking in an extremely competitive market, said SEO company were given the FTP details but unhappy with that got hold of the cpanel details and shut us out. Not a big deal until the client came back a year later tail between legs having never seem themselves feature on page 1 in any of that time, but managed to pay a whopping 4k a month for the potential privilege. *snigger*
It is very necessary to choose a best company if you want to get rank for your site on Google. A good company will take all the pains from you and will work hard to improve the site's results. It ‘s very necessary to check on the feedback or online response of the SEO company before hiring.