Use of personal pronouns in papers (research, etc.)

Back at university, I remember being told to never use personal pronouns in my dissertations. I was never given a reason, but I was told to avoid statements like

The evidence leads me to believe.

or

After researching similar techniques I had settled on ....

I remember spending a lot of time constructing sentences that used "the author" rather than "I" or similar. These are some very bad examples (as I submitted my final dissertation back in 2008).

Since then however, when I've read research papers, scientific articles and dissertations the authors have referred to themselves as "I" or "us" (when collaborated on). This seems to fly in the face of what I was told at university.

Is there a hard-and-fast rule to this, or is it down to personal preference?