Very true, and as I see it that is why you are not allowed to use an electronic calculator in the exam. By forcing students to use the slide rule, it is one way to attempt to make students learn the underlying principle, otherwise they'll have no idea how the thing works.
But at the same time, you do need to get the answers to exam question level, ie accurate to a degree or two, so I'd say double checking on an electronic calculator as practice is a good way to go.