Lily Bollinger of Bollinger was asked "When do you drink champagne?", and replied:
I only drink champagne when I'm happy, and when I'm sad. Sometimes I drink it when I'm alone. When I have company, I consider it obligatory. I trifle with it if I am not hungry and drink it when I am. Otherwise I never touch it - unless I'm thirsty.
sounds like a good rule of thumb to me! ;)