No? Yes? Sometimes?
Let's use examples.
Last week, I would have said, no, I do not need my covers to match. I own Stephen King's Dark Tower series and the covers do not match. I have a mix of paperbacks, trade paperbacks and hardcovers. The designs on the covers themselves are different. I bought the first three novels years after they were released, when I first started the series. I got others in the series at different points after their release. It didn't matter to me that the covers didn't match, I just wanted the stories. I have a couple other series like that.
Recently, I read The Eyre Affair, by Jasper Fforde and loved it. I had to find out about the other books. I believe there are currently seven books in the series with one more set to be released. I don't know why, but I really wanted the cover of the second book to match the first. Do I just like the look of it better than the others? Yes, I think so, but like The Dark Tower it shouldn't matter, I should just want the stories. I looked at different sites and checked my local bookstore. No copies of the second book were at the bookstores in town. I looked at four different sites. I found the cover I wanted, at a pretty good price too. Why was this so important to me? By the time I purchase the last book, will the editions have changed? Will the covers I have now match the others I'll get in the future? Maybe, maybe not.
I guess I don't understand why it was important to me this time around. Maybe it's just this series, just these covers. I don't know. I guess I'll find out the next time I buy a book for a series I'm reading.
* There were more than just the three covers.