Indeed. You know me, hackles up as soon as any writer, even the mighty Orwell, starts saying Here are the Rules:-
I know his get-out clause is number 6;
1. Never use a metaphor, simile, or other figure of speech that you are used to seeing in print.
2. Never use a long word where a short one will do.
3. If it is possible to cut a word out, always cut it out.
4. Never use the passive where you can use the active.
5. Never use a foreign phrase, a scientific word or a jargon word if you can think of an everyday English equivalent.
6. Break any of these rules sooner than say anything outright barbarous.
but the fact that there is a 6 suggests to me the invalidity of 1-5 as anything other than
his own stylistic choices.
Idioms (1) play an active part in rendering dialogue, and you can get some lovely bathetic or punchy effects with a well-placed idiom.
2 depends on the rhythm of your sentence, doesn't it? Ditto 3.
And 5 depends so hugely on context you just can't append 'Never' like that. I read a novel today (Jeremy Sheldon's 'The Smiling Affair'
that used a foreign phrase to convey something important about one of the characters, and it was spot on.
<Added>
That was supposed to be a closed bracket after Affair, not a wink. Damn smilies.