Just a quick one - do you ask at the time of birth or just before the christening? .. we already know who we will be asking and in some ways (especially for those who are not already parents themselves) it will be nice for the to know at the point of birth - but is etiquette normally just before the christening. thanks!
I would ask as soon as you've chosen them - if you know who you want to ask, then I'd get straight on with it. That would also give you time to pick others if they felt they couldn't, for whatever reason.