Redi-Whip and other such are very low carb, but the carbs they do have come from sugar. During Atkins, it's best to avoid sugar as much as possible. Ergo, if you can, use your own whipped cream as opposed to commercially whipped products.

As for salad dressings, I haven't found any without sugar, either. I use them sparingly, choose ones that are fatty (like blue cheese), and don't worry excessively about it since they haven't been a hindrance to losing.