Following the Trend in Business Writing
If you want to be safe, always go formal unless further evidence shows to the contrary.
A good path to take is to gauge the local climate at your organization.
For the most part, it's all about following the leader.
Do the top officers in your company write in a strictly formal tone? If they do, that's all the evidence you need to know that you need to stick with traditional business writing.
Do they write in a more casual manner? Then relaxing your writing tone a tad shouldn't be a problem.
To put it simply: the top dogs set the precedence, everyone follows.
In many technology and media companies, it isn't uncommon to find communication within the organization to border on the informal, an attitude reflected in the dress codes and relationships between the different personnel.
If you work for one of those businesses, then corresponding in a more casual manner, even through emails and official documents, may be completely acceptable.
Others, however, such as businesses in the financial and legal sector pride themselves in keeping up a strictly formal front, which you will recognize in the laced-up manner of dressing, rigid structures and the enforcement of formal writing within the organization.
Obviously, sticking to the familiar rules of conventional writing will be called for in these situations.
Whether you're writing in a formal or casual manner, as well as anything in between, it's important you compose the text using correct grammar, clear sentences and a logical structure.
My advice? Get yourself a good writing software.
It will serve to benefit your professional life in more ways than you can imagine.