Silver Trumpets is like Holy Grail because it says you "may" perform the action (i.e. you may discard), which makes it an "optional use artifact." For these types of artifacts, they are not "used" until you choose to activate the ability.
Assyrian Tribute is not optional, because it says "negate" rather than "you may negate." For these types of artifacts, as soon as you activate the artifact, it is considered a "use," even if you do not block with an Assyrian in this case. Burial Shroud is like this also.
The confusing part about whether an artifact is a "optional use artifact" is that the last line of both types says "May be used twice." That makes them all appear to be optional, but disregard the word "may" on that line. They should have worded that differently to avoid confusion. For an artifact to be optional use, the "may" must be in the sentence with the ability being used.