It depends. At the beginning of the game, when not all the treats are available, you might as well stuff the animals with all they need for all the increases, since otherwise you'd just be sitting around feeding them nothing. ^_^
And later too, even if all treats are available, it doesn't mean that Cheryl is going to see the ones you need...so might as well "feed ahead" with the ones that you DO have. But yeah, if you have all the treats available, it's fastest to go one level at a time.
Oh yeah, I didn't mean so much when you only have one treat available it's obviously best to think ahead with the days you have, but I've had the treats available since I started doing my tallies so I don't think about it that way so much (they must be unlocked pretty quickly?). And I get the point about them not selling the right ones, IT'S SO ANNOYING x-x haha But I try not to feed ahead if I have the option to work on a level : )