[Trezoid]

It really depends on the style of conversation. It's not unusual on some forums to use the quote block (linking to the the comment being replied to) but leave it completely empty to respond to the whole comment. people can click the link to see the context, but don't have to re-read the whole thing again if they're following the conversation already.

On the other hand, quoting individual lines to comment on a specific section happens plenty too.