I’m currently using Lighttpd on a VPS. Before that it was nginx, but I came back to Lighttpd because I needed CGI.
Apache’s HTTPD is mainstream. All documentations include examples for Apache and most modules are only available for Apache.
Is the effort to use something different from Apache really worth it?
I wouldn’t consider it as premature optimization. Those two webservers are good products and strong in static content. Just because Apache is widely used, doesn’t mean other products are just ‘optimization’. In my experience lighttpd and nginx are easy enough to configure and you can use fcgi bindings for most programming languages.
You should consider the functionality you need and pick a server which offers those, possibly the one which uses less resources which most likely won’t be Apache.