I always wait til the day is almost done, and send cappuccino to all my neighbours. That way, if they have requested something in particular during the day I can send them what they want, and if not, they get the highest-value food.
But, I like eric_malmquist's idea of giving food to under 30s and generic parts to over 30s.
One thing I want to know is.... if you decline a gift (say, a dragon's tooth when you never have any intention of getting the dragon decoration) does that count towards your limit of 20 gifts per day, or does declining 10 gifts still mean you can accept 20 others?