Can MySQL 5.1 read a MySQL 5.5 MyISAM file?
What could go wrong? If the way MyISAM changed the format of how something is stored in their files, it will break on 5.1. This will render your stats database on the master corrupt and unusable.

Even if the initial load went fine, it doesn't mean something won't come along later with the result listed above...a corrupt database. It's a risk, and if it can be avoided, I personally would do so.

And in your case, it can be avoided by dumping the data using mysqldump quickly and then load into the 5.1 instance.