I am copying this from Hels2007. It was written in regards to residency interviews, but applies just as well to pharm school.
"On one hand, if you interview later, there is greater likelihood they will remember how good you were (unfortunately, it is very hard to keep track of everything everyone told you when you are interviewing a bunch of people!). On the other hand, if there are some really great candidates who interview early on, the program may already make up their mind about certainly including certain people and then you may just get a half-hearted interview and lower ranking proportional to what you deserve just because the top spots are already occupied by someone else. Also, people who interview early get away a little easier, since the program is not yet sure what to expect of this year's crop of candidates. They will use early ones as benchmark and then the later ones will get somewhat harder questions."
Additionally, it's better to interview early if it's a rolling admissions school because obviously they have more spots to fill early on.