Only you know your body. Your sponsor is telling you what works for him, it may not work for you. May doctors would say that a snack is always good as long as you pick a fruit or vegetable. Some of the fruits have the natural sugar so be careful. Most vegetables are around 10 to 20 calories for an entire serving! I use celery, carrots, peppers and cucumbers. How can you go wrong with those?!?!?! Your sponsor is right about planning your food, at least have something in mind about the basic meal for the day.
Some other good advice... track your food so you know the calorie count. There are lots of on line free helpful websites. (This one is my favorite for reading some great posts.) It is easy to see if you are over your daily count or under. If you are under, then by all means have a snack!
Figure out what is best for you! Good luck!