sosaith asks, "When someone is dead or something doesn't exist anymore, should you ever use the present tense to describe them?"
You'd expect that the past tense (in any of its variations) would be used in a discussion about someone who is dead:
"When I was quite small, my father always said to me that a legions marches on its stomach," said Marcus. "so I drove my nurse half mad by insisting on food I could pack for my meals."
Similarly, places and things that no longer exist would also seem to be firmly of the past:
"The Colossus of Rhodes was a great wonder in its day," Uncle Aquila explained to Esca. "It was destroyed two centuries since, by an earthquake."
However, there is a way that past can speak in the present day: in the form of ideas that have survived, for example, as writings. In those cases, we can use the present tense:
"I wouldn't let that talkative young tribune concern you," said Uncle Aquila. "Remember what Plato says: 'Wise men speak because they have something to say; fools because they have to say something.'"
In some cases, even a memory can be enough to bring an idea into the present tense, especially if it has become, in essence, a concept:
"One who served in Judaea told me a curious thing about the Jews," said the old surgeon. "Their Temple is still at the heart of their religion, for all it was destroyed."
You can see that the general rule is that if we are talking about a dead person himself (or herself) or a thing of the past itself, we use the past tense. However, if we're talking about some current legacy or remnant of the person or thing, we may use the present tense.
Sometimes, the difference can be rather fine:
"Cicero is the finest of all the orators," said Uncle Aquila. "He lived during the time of the first Caesar."
It's pretty clear that Uncle Aquila means that of all the orators Rome has ever produced, Cicero remains the finest as of the time that Uncle Aquila is making his assertion. What is implied if Marcus' uncle uses the past tense in his statement instead?
"Cicero was the finest of all the orators," said Uncle Aquila. "He lived during the time of the first Caesar."
Does this mean that Cicero has been surpassed by an orator of Uncle Aquila's own time? Or merely that Cicero is no longer among the living? It could even imply that orators of Uncle Aquila's time aren't even worthy of being considered orators at all, and that orators are therefore a thing of the past. For clarity, the author might want to flesh out Uncle Aquila's argument a bit:
"Cicero is the finest of all the orators that Rome has ever produced," said Uncle Aquila. "He lived during the time of the first Caesar."
"Cicero was the finest of the orators of his day," said Uncle Aquila. "He lived during the time of the first Caesar. I firmly believe, however, that Quintilian surpassed him."
"Cicero was the finest of all the orators," said Uncle Aquila. "He lived during the time of the first Caesar. Today's speakers are mere schoolboys when compared with the rhetoricians of those days."