To allow all crawling you have some options. The clearest and most widely support is:
To paraphrase, it means, "All user agents have nothing disallowed, they can crawl everything." This is the version of "allow all crawling" that is listed on robotstxt.org.
Another option is to have no robots.txt file. When robots encounter a 404 error at
/robots.txt they assume that crawling is not restricted.
I would not recommend using
Allow: directives in robots.txt. Not all crawlers support them. When you have both
Disallow: directives, the longest matching rule takes precedence instead of the first or last matching rule. This drastically complicates the process. If you do use use "Allow", be sure to test your robots.txt file with a testing tool such as the one from Google.