If you aren't hungry, don't eat for eating's sake. It's a pretty good rule to stick to. It's not essential to snack and if you have plenty of veg with your meals then you'll be getting your 5 a day.
However, sometimes we just want something...not because we're hungry, just because...and then it's ok to have a piece of fruit! Better than having a cookie or packet of crisps. For me, my main trigger is boredom. I am a boredom grazer.
Getting back to the point...snacks aren't compulsory!