To put things in perspective, Ichigo wasn't necessarily needed for any Quincy plans. Specifically, the initial Ishida encounter (unleashing the Hollows on the town) could have been a way to draw out a Shinigami (any Shingami) in order to advance the Quicy plans. The fact that Ishida could use a novice Shinigami (who turned out to be much more) to further the Quincy plans only sweetened the deal. (Though, I admit that the Quincy knowing of Aizen's plans (or at least keeping track of Aizen) is a very interesting prospect, and it requires little in the way of retcon (unlike the intial idea that Aizen had been keeping track of Ichigo all along).)
Whatever the case, Ishida does have a habit of saving Ichigo. Consequently, it is possible that he has always been looking out for him under Quincy orders...or they could just be friends...but that is too boring an answer now
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