You can be totally safe if you eat plain yogurt and mix it with SF syrups or jams/jellies. I got disgusted with all the extra stuff they were putting in yogurt (like glycerin!) and started doing this a couple years ago. This way, I can eat organic yogurt, too!
Suzanne is totally right--there's no need to check the sugar grams on the yogurt, just the ingredients. Look for things that end in -ose (i.e. fructose, dextrose, etc.), "high fructose corn syrup", sugar, corn syrup, etc. Also look for fruit purees...if there's a lot of fruit, you may find that it causes cravings.
Glad you asked!