That’s a decision you should make in consultation with a physician. Since there have been no large scale clinical studies on the effects of Viagra on premature ejaculation, and since nobody has tried gaining FDA approval for this use of the drug, you would be taking Viagra off-label.
One of the main problems with taking Viagra to treat premature ejaculation (especially for men who don’t have erectile dysfunction) is that it’s expensive. No generics will be on the market in the United States until the end of this year, and the company that’s been approved to offer the generic agreed to specific terms with Pfizer (Viagra’s manufacturer) that means they won’t be selling it at the rock-bottom prices they can in markets like Western Europe, where many generics are available.