I don't think anyone applies to schools just for practice; I think if someone applied to a school, they'd genuinely go there if accepted. That said, we obviously care about certain schools more than others. In that case, I'd definitely try and get your "safety" (for lack of a better term) interviews out of the way earlier on so you can get used to the interview environment. The schools I got into were at four our of my last five interviews, while I didn't get in at all to the first three places I interviewed even though they were probably "lower-ranked." Despite being a naturally confident person, I definitely was a little jittery early on, but after my first few interviews I really became comfortable; I think my confidence and "comfortableness"--just looking like I actually belonged there at an interview--definitely left a positive impression on my interviews at the later schools.
Therefore, given the chance, schedule those interviews early.