I am planning to travel with Etihad Airways (Economy) to the US. I have a check-in bag of dimensions 72cm x 45cm x 30cm and it weighs less than 23 kgs. However, on the Etihad website, they have mentioned that the max. dimensions allowed is 158cm (70x50x38). Should I have to stick to the mentioned dimensions or I should only care about the total sum of length + breadth+ height?
Airlines do weigh checked baggage and that is the most important measurement, actual dimensions are almost never measured for 'normal' luggage. Even when they do, small differences are hardly ever a problem, especially if the overall dimensions are good.
There is, however, no guaranty a specific agent may measure and give you a harder time.
At this time there is no tradition of measuring luggage out of sight of the passenger and coming back to them with complains.
If the rules change in the future, you might be stuck with a piece of luggage that can not be used on flights.