This is an important issue. Yes, it does take some brass to request a pre-mailing review of a teacher's letter of recommendation. That seems to imply that you don't trust the teacher's judgment, writing skills, or whatever. Here's the alternative approach I took when my son was applying to college:
 
I knew that his teachers' recommendations had to go in with the EA application by early November. So, when I went to parent-teacher conferences in October (as my wife and I always did), I brought up the fact of the recommendations to those teachers whom my son had chosen to be his recommenders.
 
I thanked them for being willing to write on my son's behalf and emphasized how much my son enjoyed being in their class, etc. Then I mentioned about how difficult and competitive the Ivy/elite application process is and how crucial the recommendations are. Then I said something like this:
 
"You know, I really appreciate the amount of care that you'll be putting into your recommendation. I know that some teachers merely have a standard form letter on their computers and just fill in the student's name. I'm glad that you don't do this. The fact that you'll take the time to tell something personal about our son -- some anecdotes or special insights that make him stand out -- will go a long way in telling his schools what a special person he is."
 
By putting it this way, you can indirectly tell the teacher how to approach the recommendation. You also let them know that you're aware of the 'boilerplate' approach that some teachers use. One teacher and one counselor in my son's school used computer-generated, put-your-name-here recommendations, believe it or not. As an applicant (let alone a parent), I wouldn't have the nerve to ask a teacher if I could see his or her recommendation. Some teachers will just go ahead share their rec anyhow, mainly those teachers who want the student to know just how much they think of them. That's rare, though.
 
Choosing a recommender is serious business. Students seem to gravitate to those teachers with whom they have the best relationships. That's fine, but the teacher must understand the importance of their recommendation and not just 'toss something off. If a teacher can cite a special story or two about the student that reveals a unique insight, that's always the best approach. Also, the teacher shouldn't repeat factual information that's already stated elsewhere in the application. That's just a waste of everyone's time.
 
Parents should check out some things written by a teacher who's a potential recommendation writer (comments on papers, assignment narratives, etc.) If these simple jottings don't read well, a recommendation written by this teacher may put the student at risk. So, as I’ve mentioned before, a well-written parent rec, when solicited, can many times overcome problems posed by other, less-well-written recs.