I recently started an intern position in a new office, and was told that the dress code is business casual. In theory, it is also the case that there are "dress down Fridays", where jeans are explicitly listed as acceptable attire.
In practice, however, it is the case that most of the office usually dresses somewhere between business casual and casual. Many of my coworkers (and even some in management) will wear jeans every day to work, and t-shirts are not an uncommon sight.
This is a fairly easy going office, so I'm sure that being slightly under/overdressed wouldn't be a big problem to begin with; at a company happy hour, I was actually told by a member of another team that I must be new because "I dressed too nice".
If a dress code exists but is not strictly followed, what should one dress? I'm inclined to observe my coworkers and dress alike, but there is some significant variation in that (some dress more casual and some more formal, which makes that quite a bit harder). Nearly everyone else in the company is more senior than I am - so I have no example to set - but I believe I do have to be concerned about whatever "privileges" someone more senior may have.
This is in the US, and I work at a tech company situated in a more traditional office setting.
edit: I am in software development, and not in a customer facing position.
edit 2: this is not a duplicate of another question. This asks not about whether dressing too formally is a problem (it is), but whether following the official dress code (when it is not followed) can be perceived negatively (e.g., by looking too formal).