Word by default will search the main dictionary and custom dictionary whenever it runs a spell check. That said....
Take a look in your Word preferences under spelling. You'll see an area where it specifies whether or not to just use the main dictionary or the main dictionary and custom dictionary. If you have no custom dictionary setup, select the option to just the main dictionary.
BTW, the use of a custom dictionary is recommended if you do lots of WP with unusual words, foreign words, etc. Otherwise it's not necessary.
Regards.