I think you need to display situationally appropriate formality. That is say that your ability to write at the correct formality level, and to determine what those levels are, is important.
Consider dress code standards. Regardless of what the employee dress code says, you should know that if you're alone in your office on a weekend that shorts and a t shirt, or maybe jeans and a t shirt, are fine. Conversely you'd probably show up in your boss' office on Monday to discuss a sensitive topic looking a little bit different.
As a mid-level executive I spend a lot of time looking for good candidates for management positions. You need to know when to be informal and have a beer with your colleagues, but you also need to know that how you interact with my boss (and others) reflects upon me as your mentor.
I think the formality is fine and you should keep it up. It's not overly klunky, you're not saluting people in the hallways and snapping to attention, and when they need to pick someone to interact with higher-ups your value as someone with tact will be important.
In short, there's a difference between what you should do and what you can get away with.