In some German books, I see what I consider proper:
\int \mathrm d x
In other books, I just see this:
\int d x
Since everybody writes
$\sin(x)$ instead of just
$sin(x)$, I wonder why people write all operators upright, just not the “d”. Is it just that it is more typing (which is a lame excuse) or is there some really sound reason for that?
Please just don't say that it is “personal style”, since
$sin(x)$ would clearly considered wrong.