well with sweetness and fruitiness being some of the most important points in espresso and filter coffee, it seems to me putting sugar in will really confuse your tastebuds! Unless you can extrapolate the actual sweetness of the espresso from the amount of sugar you put in... but my tastebuds can't do maths!!
We get people who buy the jamaican blue peaberry, having never tried it before, and then drop a sugar cube in without even a taste!