Spell checkers are perhaps the most useful of the language tools on offer. Impossible spellings are automatically detected. You can set up the checker so that it detects spelling mistakes as you type or you can turn it off and activate it manually later.
It is easy enough to use the wrong word, however, typing for example sight instead of site. In such cases your spell checker will not detect any error, so you still need to check your work carefully after typing it.
Another potential problem is that you may disagree with the established criteria of the spell checker. For example, which do you prefer, cooperate or co-operate? This is largely a matter of taste. In such cases you can add what you consider to be exceptions to a local dictionary so these spellings will be accepted in the future, but only for documents edited on the same computer.