I always use the past tense in this situation (reported speech). For students, it will always be right and you don't have to think about it.
I agree that backshifting is always correct, and I always recommend that my learners backshift.
The phenomenon Mike is taking about hasn't reached Australia yet,
That surprises me. Henry Sweet wrote as long ago as 1900, "[...] in such a sentence as
the ancients did not know that Africa ..an island, we hesitate whether to use
was or
is." Jespersen (1933) wrote: "[...] when the idea of a universal truth is quite obvious, the tense may be unshifted:
We learnt at school that 2 and 2 is 4."
It is true that not everybody will regard the beauty of a particular speaker as a 'universal truth' but Palmer (1974) makes no restriction: "The present tense form of the original statement can be retained even with a past tense of reporting:
He said he likes chocolate. He said he's reading 'Vanity Fair'.
It is interesting that in the earlier version of his book (1965), Palmer said that non-backshifting was 'rare'. My own suspicion (and that's all it is) is that between 1965 and 1974 Palmer realised that it was not rare; it was in fact quite common - and correct. I taught obligatory backshifting when I started in TEFL in the late 1960s, because that was what most coursebooks and student grammars at the time prescribed. Within a short time, I had realised that backshifting was not essential for 'universal truths'. By the mid 1970s, I had read enough to confirm my own belief that backshifting was not essential for any situation that was still valid at the time of reporting.
Sweet, Henry (1900:70) The History of Language, London: Dent
Jespersen, Otto (1933.261) Essentials of English Grammar, London: Allen & Unwin
Palmer, F R (1974.45) The English Verb, London: Longman
Palmer, F R (1965.71) A Linguistic Study of the English Verb. London, Longman
and seems to be unnecessarily confusing.
I don't entirely agree. Universal backshifting may appear simpler, but the fact that many native speakers, of BrE at least, do not backshift when the situation reported still holds at the time of reporting is not particularly confusing, in my opinion.