A Search Engine Watch Forums thread asks whether it's useful (or detrimental) to vary your anchor text. Should anchor text be differentiated?
The answer is yes. As AussieWebmaster says, "always vary your anchor text."
Is there a sweet spot? Not really. But what you should do is make sure that the anchor text is well optimized (you can use a thesaurus, for example, to find other reasonable search terms) and that you're not too spammy. The links should look natural, not unnatural.
Forum discussion continues at Search Engine Watch Forums.