Abbreviations can be used frequently
Abbreviations should be used when they are likely to be understood by the audience.
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.
Follow the style guide.
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.
Your favorite style guide