Ahhhh, the eternal question!
As far as I was concerned I would help my DS1 (and now DS2, taking it this year) to develop the techniques and background knowledge they needed to pass and succeed at grammar. If that meant working hard with me, then that's what happened. I was not happy with the two comps in my catchment area(one in special measures and the other 'improving' but with some way to go).
The problem is you really do have no idea what the standard will be the year your DC takes the test. Our joint philosophy was "If you want to have a choice you have to do the work." My DC's often quote it to me now!Thankfully both my DC's have been happy to do the prep 99% of the time.
I decided on the format of work: reading, vocab improvement, practice papers, basic maths, comprehensions etc. which we did bits and pieces of each week. It was not that I thought my DS1 needed tutoring because he was 'not quite bright enough to pass on his own merits' it was just I reckoned everyone else would be tutoring their kids and he might lose out because I had not structured his time effectively to prep for the exam. As long as other people are going to tutor their kids, then I was going to tutor mine. If you can't beat 'em...and make no mistake you will be in the lap of the gods if you don't give them any tutoring, then join 'em. Enjoy the journey and try to make it fun. I did not worry at all what, or how much, others were doing. I knew my child and his capabilities and never cast a thought to what other people thought was 'acceptable' or 'not acceptable.'
At the end of the day it is your DC who has to open that letter, deal with the contents (hopefully happy!) and make that choice. No-one likes to talk about it but believe me there will be parents having their kids tutored up to the hilt while glibly saying "Oh, we have done nothing!"
I did the prep with my child and my conscience is clear. He is off to Pates this year and he cannot wait
Now ....just DS2 to go. Am I nervous....like a wreck!!!!!