Do you need a robots.txt file for the crawlers to index your site, or it is just to disallow any content that you don't want crawlers to index?
If you don't have a robots.txt your website will get indexed. The robots.txt file are just some directions for (nice) crawlers on what not to crawl.
Please also note that there are aggressive crawlers out there that just ignore it.
You may also find 404's in your error logs if you don't have a robots.txt in place, if you want to avoid those just create a blank file named robots.txt in your sites root.