use common sense when snacking.
a bowl of ice cream? fine as a snack.
a carton of ice cream with half a bottle of hersheys and a jar of peanut butter? probably not so fine.
assuming your snack foods do not make up the majority of your diet, an occasional deviation from your diet is hardly going to negatively affect what progress youve made/will make. whats going to determine your progress is what you are eating the other 95% of the time.
you could even time a high carb/low fat snack time after a weight workout and worry even less about any negative effects.
of course the typical popcorn/celery stalks/fruit cups/etc are fine. regularly eating a 1 pound bag of skittles might not be the best idea.