I´ve heard this, but let´s see what the native speakers have to say about it.

I would say something like "your opinion may lie anywhere between these two extremes", that's anyhow what they want to say :)

I think "rating things on a scale" is the most common way of expressing this. (See the nearly 6 million hits for this at Google)

How do you rate this year's fair? For the following points rate your satisfaction on a scale from 1 to 6, with 1 being "very satisfied" and 6 being "not satisfied".

I would be inclined to combine the sentences together, from "1 bedeutet „sehr zufrieden“, 6 bedeutet „nicht zufrieden“. Dazwischen können Sie abstufen." to something along the lines of "Please use a grading scale from numbers 1 to 6, 1 being "very satisfied", 6 being "very dissatisfied". I believe using the wording of "scale from numbers 1 to 6" would be ample indication that the customer can use 2,3,4 or 5 to indicate their satisfaction accordingly, and thereby transfer the meaning of "dazwischen abstufen".

In the UK, for Market Research purposes, one invariably sees marketing material asking the customer to rate their experiences using variations of the word "satisfied"; Very satisifed, quite satisfied, satisfied, neither satisfied nor dissatisfied, dissatisfied, very dissatisfied. I am unsure whether you would have the freedom to do this instead of using a number grading scale, but it would relate to numbers 1 - 6 accordingly.

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Note added at 6 hrs (2011-03-21 14:29:52 GMT)
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As orla has suggested, seconded by Maurite, change "Please use a grading scale of 1 to 6" to "please rate on a scale of 1 to 6"