Search engine optimisation is usually divided into on-page and off-page SEO. This basically means things which are done to your website (changes and updates) and things which are done outside of your website (link building, etc).
Some SEO companies will just deal with on-page SEO so will need to have control of your website. Other SEO companies will just deal with off-page SEO so won't need access to your website.
But, if you're thinking of hiring either of these types of companies, you're probably wasting every single penny that you spend. To see the full benefits of search engine optimisation you need both types.
Make sure you understand what is required of a search engine optimiser. At OpenGlobal, we do not take on search engine optimisation contracts without full website management. There have been numerous cases where companies have got into problems using a separate SEO consultant from their website management company. One company blames the other for any problems and the client doesn't understand the technology enough to know who is really to blame.
For this reason, we choose not to take on these sorts of arrangements. From our perspective, the difficulties of trying to optimise a website effectively using somebody else's software just isn't cost effective.
Contact us if you would like to find out about complete search engine optimisation packages.