I don't think Singulair affects your blood sugar levels. Which is pretty rare, for asthma drugs.
Both beta-2-agonists (like albuterol/ventolin, serevent and foradil/oxeze,) and corticosteroids (like prednisone, flovent and pulmicort) have the nasty side effect of jerking around your blood sugar.
Beta-2-agonists can make your blood sugar drop by increasing your cellular metabolism. Doesn't mean you can't use them, but it means you should be careful, especially since it says right in the monograph that it can aggravate pre-existing diabetes mellitus and ketoacidosis. (then again, if you're in diabetic ketoacidosis, you've got slightly bigger problems than bronchoconstriction.)
Corticosteroids do practically the opposite -- they make more glucose available for metabolism and re-distribute fats within the human body. This can make your blood sugar rise. It won't make it shoot through the roof -- actually it probably shouldn't make it fluctuate all that much -- but just be careful until you learn how the drug affects you.
And if you're only taking Singulair, then yay, because you don't have to worry about any of this.