Once you've introduce an acronym / initialism should you always use it?

When writing a large document with technical terminology (in this example I'll use 'Content Management System') I would use the full term itself and alongside introduce the acronym / initialism:

...When using your Content Management System (CMS) you should first....

However, once I've done this should I always refer to it by the initialism throughout the document, or is it advised to refer to the term by its full name at periods throughout the text?

I don't want people getting confused by the term if they repeatedly come across it and not understanding what it means, so would be tempted to occasionally reuse the full 'Content Management System' wording in order to associate that with the initalism again, but then equally I don't want people thinking "they've already told us what CMS means, why are they using the full words again?"

Should I just stick with the initialism throughout?

You should just use the abbreviation in almost every case the rest of the way through. It is a technical document and you have already given context on the abbreviation. Not only is the user expecting you to use it but by using it you are making the user more familiar with the term. By jumping back and forth you are confusing the reader and you are making them read more.

The case where it would be used again is if you were discuss what a Content Management System is or something like that.

Much depends on two things: (1) the level of familiarity you expect most members of your probable audience to have with the initialism; and (2) the length of the document.

If most of your readers already know what the initialism stands for, you are spelling it out in the first instance primarily as a courtesy to the relatively few who are behind the curve. But if the initialism is likely to be unfamiliar to a large number of your readers, the potential usefulness of reminding them of what it stands for becomes a valid consideration. This is especially true if the initialism appears early in the document but not again for several pages, sections, or chapters.

This is where the length of the document becomes a crucial factor. I understand your desire to avoid antagonizing elephant-memoried readers by pelting them with repetitions of the spelled-out term, but no reader should be forced to hunt through dozens or hundreds of pages to find the meaning of an unusual initialism.

Likewise, if a document is likely to be read incompletely and from different starting points, it doesn't make sense to assume that readers have already seen the first occurrence of the initialism and its spelled-out equivalent. In such a case I would be inclined to repeat the spelled-out form at its first occurrence in each major section of the document—or at whatever level of frequency seems most likely to serve the interests of the greatest number of your readers.

Something similar happens with proper names: It is standard journalistic practice to identify a person quoted or cited in a story by first and last name and job title, and by last name thereafter. Thus a person identified on first appearance as "Robert Jennings, a pipe fabricator at Corncob Enterprises," thereafter becomes "Jennings." But if Jennings disappears from a lengthy feature for an extended period, only to reemerge five pages later as plain old "Jennings," the author isn't doing readers any favors by expecting them to recall that Jennings is the pipe fabricator from page 3.

As a practical matter, you can reduce the likelihood that repetitions of the spelled-out form of an initialism will grate on readers who know perfectly well what the intialism means, by avoiding the "consecutive translation" approach for all reintroductions of the long form. That is, after referring to "Content Management System (CMS)" the first time the concept arises, you can reintroduce the term in subsequent sections as simply "Content Management System" (on first reoccurrence) and then refer to it as "CMS" without further elaboration throughout the remainder of the section.

Again, the best way to determine when and whether to repeat the spelling out of an initialism is simply to ask yourself what approach makes the most sense for the largest number of your readers.