Abbreviations can be used frequently
Abbreviations should be used when they are likely to be understood by the audience.
Background
Furia (Ten myths about technical writing) talks about spelling out abbreviations on first use and then using them as needed and how they should be used as appropriate to the audience and subject matter.
Survival tips
Follow the style guide.
Commentary
This seems like a matter of style and audience expectations. If readers expect an abbreviation, spelling it out will seem awkward. Likewise converting frequent phrases to abbreviations can make the text much less readable.
Mentions
References
Your favorite style guide
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