If you're working in Stickies and you're not sure you've spelled a word correctly, just Control-click on the word and a contextual menu will appear.

At the top of this menu will be choices for what it believes to be the proper spelling of that word (if it's actually misspelled and it recognizes the word in the first place).

If you agree, just move your cursor over that word, release your mouse button, and your misspelled word will be replaced. Mighty handy!