[susan53]

This is not something confined to the use of the past perfect form, but is a general rule governing backshift of all verbs in reported speech.

First of all, yes - when the time of the reported event is still current - ie the same at the time of reporting as at the time of speaking (as in your example - the event was past at the time of speaking and remains past at the time of reporting), then verb backshift often doesn't happen. An example for a future event : My boss wants a colleague (Chris) and I to visit a client on Friday. I talk to Chris and she says :
I'll be in Rome for a meeting on Friday.
Ten minutes later, I report back to the boss. I would almost certainly say :
We'd better make it tomorrow. Chris told me she'll be in Rome for a meeting on Friday.
without backshifting will to would. In this situation, backshift would tend to suggest some doubt about the occurrence of the event - ie that it might no longer necessarily be current:
Well, Chris told me she'd be in Rome for a meeting, but if it's really urgent, I could ask her to postpone it to Monday.
Backshift becomes obligatory however, when the time changes - for example if I report the event on Friday, it's no longer still future but now present :
Boss - I thought you and Chris were going to visit XXX today?
Me - I had to fix the appointment for Monday because Chris told me she would be in Rome for a meeting all day today

Notice this is not in any way "informal". The style of these utterances is completely neutral. It's a matter of the meaning the speaker wants to express. And the mere fact of being "written" wouldn't change anything : if instead of speaking to my boss I told her about the situation in an email, the use of the verbs would remain the same.

In more formal written or spoken English though, there might well be a greater tendency to use backshift when there was the option of either form. Here are five random examples that came out of a concordancer using a corpus of written data only (UK and US). In all cases backshift is optional, but in all cases it is used :

You couldn't have meant it when you said that you wanted to kill her
Madden... said that tomorrow he would go to the Bronx bank
Jubal said that his night-sight probably came from ....
Rachel said that schools and synagogues occupied most of the buildings
It said that Lamont had responded to many of its suggestions

On the other hand, here are some from a corpus of UK spoken data - again using only examples where backshift is clearly optional :

Well, I said that's understandable
Sue told me the other day that er she won't come out
No he told me ... they hadn't got a bed for him,
I told him as long as he shouts just United that's alright
I told you I went and looked that up not long ago,

One example with backshift as opposed to four without. So yes, in spoken English avoiding backshift probably is more common than in written English. But as with many things, it's a tendency and stylistic usage, not a grammatical rule. Ultimately, all other things being equal, it will always depend on the meaning being expressed - whether the speaker/writer wishes the event to be presented as still current or not.