I couldn't give you a definitive answer without doing some experimenting myself, but generally in computing contexts, AND has a higher associativity than OR, which means that ANDs are grouped before ORs. This is true for XPath as well, so my guess is that in your example, the latter version is the way the logic would be evaluated.
The condition builder dialog in InfoPath is ambiguous when you start mixing ANDs and ORs, so I would suggest that rather than doing so and causing yourself or someone else some confusion and headaches down the road, that you either:
-Handwrite one large expression, combining the ANDs and ORs the way you want with parentheses, or...
-Arrange your condition so that you are only using either AND or OR in the dropdowns, and use handwritten expressions for the sub-conditions where necessary, e.g.:
A is blank [AND]<---dropdown
(B != '' or C != '')
This way there is no ambiguity, and no confusion.