depends where you are and what it costs but you have as much right to a charging point as anyone else, so do what you want.
If you are shopping for a couple of hours you could put in enough to cover the costs of driving but anything over 40p a kw, its probably cheaper on petrol, so only really worth while if you are emissions conscious where you'll be driving and would rather be on electric.
The cost of some of the chargers around here it is cheaper to use the petrol tocharge the car
