I'm with Peter on this (and have deliberately not retyped the subject line ;) ) and have said it in the past - including all of the previous post, especially when it is a lengthy one with many photos, has much more of an impact on readability (and knocks on to storage and performance) than the subject line.

I change subject line when there is something specific in my reply I want to highlight, or my response might have somehow changed the direction of a discussion but there are times when I simply don't find it necessary.

Quoting an entire post, replete with photos, I don't think is ever necessary - pick the bits you want to address and include those if you like, but don't include the whole thing. Which, unfortunately, is the default behaviour of the software...