J.C.
Hey Sara
We used a week-long rule (yes, we expanded the 4-day one!), not because of history of allergies in the family (though there are a few of those), but because it is our first child, and we wanted to be ABSOLUTELY sure. Just like our daughter isn't around smoking, and just like we washed her clothes in separate detergent, we wanted to be VERY sure she wasn't allergic to anything.
And so far, she's not. BUT, she *was* picky when we introduced some of the food, and didn't take very well to baby food (but then again, neither did I at that age). She's 2 now, and she's particular about certain foods even now - she very rarely eats meat or other stringy foods - so you'll want to watch for that as well. (Textures, I mean)
And I think it's VERY practical - not everyone can drink milk, not everyone can have wheat products, one of her grandmothers is slightly allergic to strawberries, and I have a friend who can't drink most soda (allergic to caffiene), and another who can't have chocolate (poor her!). Now, I know most of that stuff isn't intro'd for a while, but it can be important to know, ESPECIALLY with the gluten/caesin (wheat/milk) thing! Allergies might be mild in some people, but cause anaphalactic shock in others from just the SLIGHTEST amount.
I'd be careful, just to be sure!
Jes