if you want to allow all pages index by all search engines than just upload a blank robots.txt file in root.
OK.... I have neither uploaded a blank robots file nor filled one, still my website's pages are indexed in google..!!
Only the robots of Google, Yahoo! and MSN follow it. Robots can ignore robots.txt file especially malware robots that scan the web for security vulnerabilities. Also, email harvesters used by spammers will pay no attention to the robots.txt file. If a bad robot is operating from single IP address, then its access can be blocked via server config or a network firewall. If copies of the robot operate from different IP address, then advanced firewall rules configuration can be used that automatically block access to IP addresses that make many connections; but it can hit good robots also
Best way to block spammy robots is blocking their ips using htaccess. You can find the ips of bad robots from your website admin panel's traffic log..