Capitalization of explanation of abbreviations

I'm writing a rather tehnical report with a lot of abbreviations such as QoS (quality of service), AES (Advanced Encryption Standard) and HMAC (Hash-based Message Authentication Code).

When should the explanation of the abbreviation be capitalized? For example, should it be quality of service or Quality of Service? How does one know what to use for the vast majority of technical abbreviations?

According to the Wikipedia Manual of Style, these are some rules on the matter (I'll paste it):

When showing the source of an acronym, initialism, or syllabic abbreviation, emphasizing the letters that make up the acronym is undesirable:

Incorrect: FOREX (FOReign EXchange)
Incorrect: FOREX (foreign exchange)
Correct: FOREX (foreign exchange)

If it is necessary to do so, for example, to indicate the etymology, use italics: FOREX (from "foreign exchange")

Specifically, do not apply initial capitals in a full term that is a common noun just because capitals are used in the abbreviation.

Incorrect (not a name/proper noun): We used Digital Scanning (DS) technology
Correct: We used digital scanning (DS) technology
Correct (name/proper noun): produced by the British Broadcasting Corporation (BBC)

So: don't capitalize if the words are "common" and capitalize if the acronym refers to proper names. See also page 81 in this document.