I always navigate 100% IFR (GPS/VOR/DME/ADF) and it is easy to estimate the required offset for the actual wind.
The typical error in Form 214 is such that the accuracy of the slide rule (if used correctly) exceeds the accuracy of the original data, and a rule of thumb is just as good.
I have never used my slide rule for any real flying. It is a relic from WW1, and as it can only really be used on the ground if one really wants accurate headings (how do you get accurate wind data?) one can use an electronic equivalent.