Spell checkers will catch some kinds of errors, but not all. For example, they tend to miss homonyms -- words which are pronounced the same way but spelled differently, such as site/ sight, there/ their/ they're, and its/ it's. Most spell-checkers, for example, would report no error in the following sentence, despite the fact that there are three serious spelling mistakes:
Their looking for a new sight where the gopher can build it's home.
The joint influence of British and American spelling on Canadian usage has provided an additional challenge to Canadian students: Canadians tend to follow standard British spelling for certain words (axe, cheque), to follow American spelling for others (connection, tire), and to allow either for yet more (programme/ program, labour/ labor, neighbour/ neighbor). The important thing to remember is to be consistent in usage and to follow a regular pattern when you spell. Don't mix neighbour with labor, for example. Choose one or the other pattern, and follow it closely. The best way to avoid problems with mixed British and American spelling is to keep a dictionary handy that shows Canadian usage.
Although spelling correctly is largely a matter of practice and the common-sense use of reference materials, there are four standard spelling rules. Although each has exceptions, if you study these rules carefully, you will be able to avoid most common errors, even without a spell-checker.