I read an interesting post this morning about using active and passive voice; the dilemma the technical writer was having was that she was writing SOPs and wanted to use active voice, but the engineer (the subject matter expert) was vehemently opposed. Why? Seems that, for some reason, this engineer had it in his head that passive voice is preferable to active. So, instead of writing "Perform the maintenance" (as in "you" perform the maintenance, which is active), the engineer was insisting that it be written as "The maintenance should be performed" (clearly passive) and problematic, as it doesn't indicate who should be doing the maintenance. Active voice should be used when it's important to emphasize the actor, while passive should be used when it's more important to emphasize the action. Clearly, when one is writing SOPs, using active voice is preferable.
Use active when a particular action is required of someone or when the user/writer is expected to complete a particular action; use passive when it's less important who or what completed the action and more important that the action was completed. For instance, I can write:
"I blew up the lab" (clearly I want to show that I'm responsible, which is active voice) or "The lab was blown up" (passive voice, showing that it's unimportant or unknown who blew up the lab). The point really is to remember that using active or passive voice in technical writing should be a conscious choice that writers make depending on the situation and what should be emphasized and/or de-emphasized.