The simple answer is: "Sie" is always appropriate unless you are addressing a child.
However, there are a lot of situations where the "Du" is more common today; this includes most usages in forums, blogs or social networks, leading to the belief that it is accepted for general use.
Some companies use it as a form of corporate identity. Ikea and Apple come to mind with their notorious "Du" in ads and I think also in the quick start manual of Apple devices (the short paper thing that is in the box). The real manual, however, addresses the user as "Sie", so even in "Du" companies, it seems that this is more appropriate in operation manuals.
Know your audience. In most fields, "Du" is not apprioriate in manuals. Exceptions are typically lifestyle products, but even there you'll find the "Sie" in the actual manuals. When addressing children, "Du" is fine. This can also be used to distinguish between parts for the children and parts for the parents.
German board game rules often employ a form that has not been mentioned yet:
Der Spielplan wird wie in der folgenden Illustration aufgebaut. [...] Jeder Spieler erhält 5 Gold und 2 Aktionskarten.
This is a neutral form that uses neither "Du" or "Sie", nor "man". It is not really appropriate for a technical manual, however ("Der Benutzer drückt auf die PROGRAM-Taste." (; ). Interestingly, in a FAQ for such a game, you'll likely encounter "man": "Kann man nach einem Spielzug eine Karte abwerfen?"