A small keepalive helps prevent browsers from trying to open too many connections and reuse existing ones more efficiently from what I have seen. Nginx handles connections more efficiently than apache so it doesn't hurt. But even Apache can benefit from a couple seconds of keepalive to get less thrashing.

You can use the optional second part the keepalive_timeout setting in nginx to send timeout hints to modern browsers, ie.

   keepalive_timeout 10 10;

Some servers like litespeed have the easy ability to do keepalive for static content (ie. a series of images) and then connection close for dynamic. This behavior can be emulated by nginx with the right configuration.