eat when you're hungry and be guided by your BG meter.
Really as to whether you should eat before/after your workout depends somewhat on how intensive your workout is.
Useful info to know: An elevated heart (>80% VO2Max) rate causes a stress response which will trigger your liver to release glucose into the blood stream.
Basically if your workout is super intensive then you are likely to get a blood glucose spike during/after your workout, so it's worth waiting for your BGs to normalise afterwards before you eat (any carbs you eat will have a smaller impact on any blood glucose rise too, as your muscles and liver will be re-stocking their energy stores and help mop up any circulating glucose)
If it's a fairly steady aerobic workout, then you can eat before, as the exercise will help lower any potential blood glucose rise after eating. (Your working muscles will draw out some of the circulating glucose in order to fuel your activity)
If your snack or meal is zero carb, then obviously it really doesn't matter when you eat.