It has been my experience that even with fairly large pressure drops, the partial vaporization that takes place in the adiabatic flashing of a liquid is relatively small ... 20 to 30 % or less. Thus, the amount of flashing that occurs as the feed enters a distillation column is probably quite small indeed in most cases. Thus, it is conservative to assume that no flashing takes place. After all, an adiabatic flash is simply one theoretical equilibrium step and, in most cases, if we can calculate the number of theoretical stages required in a distillation column to that accuracy (i.e., plus or minus 1 stage), we are quite happy.
In some cases, if a quite large pressure drop exists between the distillation feed liquid and the distillation column operating pressure, the feed is flashed in a pressure drum ahead of the distillation column. The resulting separated liquid and vapor stream are then fed into the distillation column at two separate entry points.
I hope this is helpful to you.