So back to the original question. If the drink cart has already gone by and the flight attendants are "behind the curtain" then sure, ring the button. Just be polite and remember your please and thank you. It will go a long way.

When not to ring the button: When the attendant is obviously very busy at the moment dealing with 100 other things. For example. Inflight medical emergency is taking place. Call button is pressed. Passenger wants a coffee. Passenger is irritated when told he will have to hold on. True story.

As far as coming up to the galley. If the curtain is closed it is closed for a reason. Flight attendants have to eat too and are trying to do it out of view of the customers.