Having read this thread again I just want to explain exactly what I meant. Sorry I came across as though I was condoning bad grammar in teachers, actually it makes me cringe and personally I'm very careful to check grammar and spelling. But occasionally I may look at what I've written on the white board, in the heat of a lesson, and realise to my horror I've made a small mistake, as anyone would.
I only have time to read Mumsnet occasionally as I'm a full-time teacher and have two young primary-aged children myself, and I do so as a Mum because I often find it interesting and useful. But when I picked up on the vibe about marking a teacher's work it touched a raw nerve. I work in an outstanding school and pride myself as a really good, hardworking teacher. I strive to bring out the best in my pupils, as do all the teachers in the school.
The new Creative Curriculum is all about child-led learning, specifically to bring out children's excitement about learning and work because they want to (eg ask to write a story/poem about the topic), rather than being told to. Children tend to write and write without many boundaries or frequent interjections by the teacher, so many mistakes are not corrected in the spirit of allowing their creative juices to flow. It works amazingly well and the standard of their work, including their spelling, is vastly superior to the type of work carried out 20 years ago, when I first started teaching because they write so much more now, and phonics is a separate lesson in itself when you would take time to correct errors. I find that children demand to find out correct spellings themselves to add to the importance they themselves put on their work.
However, it all comes at a cost and that is usually in the form of stressed, over-worked teachers. Unfortunately I have witnessed several excellent, inspiring teachers buckle under the pressure. 
What I meant about petty complaints really meant please don't add to the pressure by complaining, when really and truly you could explain to the child the correct word yourself.
Just one other point. Some of you are shockingly patronizing and when I discussed what was said in this chat yesterday in the staffroom, the whole staff was appalled that there were even teachers giving a voive to complain agaist teachers. 
You added a comment yesterday morning Feenie, so you must be at home. Maybe when you return to teaching you'll realise that most of the dead-wood in teaching, which I have witnessed enough of, has mostly gone. And as for TAs, well they are vitually doing the jobs of teachers themselves with shockingly little pay.
I'm really upset at the strong resistance to my suggesting you don't complain. I'm afraid it is rather naive to assume that a parent complaining is not a topic of staffroom discussion and annoyance.