I've found that people don't really notice or care when IE shows them square corners.
The thing about IE6/7 support is that it's a complete known quantity at this point. The problems & workarounds are well documented. There's tons of experienced HTML coders out there that know IE backwards and forwards. CSS frameworks like OOCSS include built-in workarounds.
So if IE support doubles the cost, it must either be something very complex, or a real nickle-and-dime project. (Either that or the webdev doesn't really what they're doing.)
That's not just an IE6 problem. IE8 has full CSS 2.1 support but can't do css3 rounded corners, opacity, or shadows. You can sometimes fake it with -ms-filter and CSS expressions (htc files) but those work just as well on 6 and 7.
That said, I'm happy to drop IE 6 where not required by the client - 8 may not have html5 or CSS 3 features, but at least it's relatively bug free.
Rounded corners are already out fashioned. If a client want IE6 support I take it as a good challenge, and do something nice under this heavy constraint. I'll tell them upfront that they won't have cheap drag and drop and that new stuff.
Trying to build a responsive, dynamic webapp, that is identical in IE6 to a modern browser can easily add a lot of time. Just as an example, producing all the graphics for rounded corners, rather than being able to do them in CSS.