Ken,
Without looking at the internals of two different versions you can never be sure, but I would caution that you may very well run into problems that will not be immediately evident.
When we do minor revisions (1.1.X) to releases, there are always changes to the code base, but there may or may not be DB changes as well. Because the change may be in a supporting table, you might not run into it right away, but it is lurking out there like a data landmine.
If you have been on 5.1.7 for some time and need to go back to 5.1.6, realize that unless specifically documented otherwise, code isn't tested for backward compatibility, and you could be fine, or run into unexpected results.
Sorry I don't have better news for you.
