Dave
About 80% of my work is sash windows, the best i've found is a rectangular vikan with the plastic stock cut off at each end & along the top. this allows the brissles to get right into the corners & under the top sash. they also hold enough water to give a good flush under the top sash (mega important). using the correct brush & technique you can clean these windows with no probs.
When i first started i was sold oval vikans (the old style) & would nearly always get runs from under the sash & never get into the corners. like you say, from one angle they can look perfect, look closer & they can be awful.
Tony