This step is even more crucial than the translation itself. The reason is obvious: whereas the potentially poor performance in the previous stages can be fixed later, editing/proofreading is often the last stage when a native speaker is involved in the process. Hence poor translations still can be improved by an attentive and qualified editor, having someone, who is not fully capable in charge of this task, can ruin the work of previous participants.
My work experience as an editor/proofreader has taught me that there is always something that can be improved. Not always because of inappropriate work of the translator or his/her lack of attention, but simply because two pairs of eyes can see more than just a single one.
Comparing source and target text to reveal inaccurately translated words, missing parts of sentences, grammar mistakes, and to generally spot mistakes, is my greatest strength. I have an eye for everything that does not fit into the source/target (bilingual) view and somehow every mistake sets an alarm off in my head. No mistake can be hidden from me and my attention. If you need a perfect result, involve me into your revision efforts.