With reference to families that chose not to tutor: last year our daughter sat for four schools often mentioned on these threads and got four offers. She went to a middle-range prep and was in the top third of the class, but by no means a prodigy. We did not tutor, but just did some practice papers over the xmas holidays so she would be familiar with the types of questions she would encounter on the exams, and had some practice answering questions under a time limit. We also had a family friend conduct a practice interview with her. That's it.
If you are coming from a state primary and hoping to move to a selective independent, then some tutoring might be necessary to bridge a gap, but otherwise, I would think very carefully about whether the extra time spent with a tutor (as opposed to playing, doing sport, resting), the money spent, and the extra pressure and stress are worth it, especially as a child who only gets a place at a certain school after lots of tutoring might have to continue the tutoring in secondary just to keep up.
Of course the system isn't perfect, but on the whole I think it works out most of the time, and I believe the schools are constantly honing their selection processes to make them as fair and correct as possible. After all, it's not in their interest to admit pupils who will not thrive there.
From my daughter's prep, a few were initially disappointed with the offers they received, but a year on, all are happy where they ended up. My advice would be: think very carefully if the schools you are looking at are really suited to your child (and isn't on your list just because of its prestige), tutor only if you think it's really necessary to close an obvious gap but not just because you think everyone else is (it's not true that everyone else is tutoring!), don't sit too many exams (just choose a small number very carefully, and make them all schools that you would genuinely be happy for your child to attend, even if it's your "safety" school), and most of all, try not to let your own anxieties about the process filter down to your child.
I know it's easy for me to say, since we're already out the other end, but a year from now you will be too. I remember very well how it felt last year, and just wanted to send you all reassurance that, in almost all cases, whatever happens will be for the best, even if it doesn't feel like it now.