@keely: I work within Big 4 and I personally think following up after an interview is a nice courtesy. I would recommend email, since mail will not be timely (kind of takes away from the whole point of following up), and they may not even receive it before they make decisions (I am in audit and only go into the office occasionally, but even managers only check their mail when they are expecting something, and tax people are in the office more and I have asked friends and even they only check once in a while).
Short answer, go for an email.
Also, to your point, the follow up is more of a professional courtesy than a decision maker. In the rarest of rare circumstances will two people be so equal in every regard that the decision comes down to whether or not one sent a card/follow up. Typically, we make offers the day of after second round interviews or the day after (this is also customary with the other firms for the majority of their offers in my market). Of course, in a smaller market this may differ since they have to hit their quotas more precisely. Regardless, email follow up is ideal. A card may be nice, but it's just not done, and when it comes down to it, it's not timely. Decision made.
Focus on kicking butt in your interview. At the end of the day, .02 on your GPA is more likely decide whether you will be hired/not hired than any birthday card you may send. Typically, after office interviews the entire team has a consensus meeting and they make ratings immediately and the offers are finalized soon afterwards.
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