This recommendation was useful on JDK 7 but as long as you're on JDK 8, you can get very close to the 32GB boundary and not exceed the limit for compressed oops (for example, 31900m should be fine). In Elasticsearch 2.2.0, we now print on startup whether or not you've crossed the threshold.
However, do note that your goal should be to minimize the size of the heap, not maximize it.