Before attending any yoga class it is advisable not to have a meal or too much fluid within a couple of hours before a class.  Most postures have a significant component of using abdominal muscles.  The more intensely you physically work, the more you will regret having something in your tummy.   So this means avoiding your normal size meals or drink.  Basically if you can feel the mass (food or water) in your tummy before a class, you have too much.

However for any of the evening classes, when you may be low on energy, especially coming straight for your daily work activity, having a small handful of nuts or sultanas (or similar) will keep you going.

Even with the Relaxation Class or Towards Meditation Class, if you have a full tummy, it can be very distracting, and you are more likely to fall asleep then to gain the full benefit of the practices.