Screen captures from the 1992 Special Letterbox Collector's Edition VHSes:

Ep IV: http://originaltrilogy.com/forum/topic.cfm/Smear-free-93/post/481961/#TopicPost481961

Ep V:

^ Somewhere between that last shot and this one, there was a bit of a drop in quality. It's getting harder to seperate the edges of the picture from the "black bars". Seems like this quality loss persists through the rest of the movie (and gets slightly worse).

Ep VI:

^ Example of framing and subs. The quality is a bit worse than ESB had been getting.

^ The quality improves! I can find the borders again!