It is not a good idea. Extra click/login can turn away shoppers unless they are so hooked on and are convinced that there is a substantial incentive to login. This happens rarely (mostly with bigger players).
One of the factors determining conversion rates is the kind of products sold on the site. The average across product segments is around 2-3%, but can be as high as 30% for some segments. Some example product segments that get high conversion rates – Flowers, Office products, Tickets and Custom/Niche Apparel. Also, catalog retailers and home shopping networks tend to have higher online conversion rates.
Sites like Proflowers.com, LandsEnd.com, Blair.com and Tickets.com consistently rank high on conversion rates. Amazon and eBay in general rank among top 5 in almost all segments.