Not for me, but a lot of client expect the see the same markup they would see if older browser weren't supported.
Really? Who are you coding for?
My clients barely know what a browser is.
And still no dilemma: one cannot use space-age techniques with stone-age browsers. Nobody anywhere has done that, so any clients who expect modern markup with ancient browsers is dreaming, because they've never seen it.
It is our job as professionals to explain that to clients (if you somehow get one who can actually read HTML and CSS, lawlz).
On the other hand when clients make up what their expectations are for "satisfactory" work they invoke terms they heard from "being educated by other web developers/designers",whose work they probably understand less than 3rd grade geometry.
You have to show them real examples on real browsers. I had a client who used IE7 and I had to show him exactly why the font he wanted could not go on his page (he had bought Humanist 251? for his logo print design) and showed him what the font stack looked like on Windows and on Linux, and told him Macs would look different as well.
Having some demo pages pre-prepared is a good idea, with browsershots for any OSes you don't have. Then whip out your iWhatever (or your client's iWhatever) and show how different the page looks there. Yeah, you can't use print math, but showing is telling.
Possibly one technique you can use for stubborn dreamers is to take all those trendy web5.0 Designer sites (which are using all that HTML5 and CSS3 just everywhere for everything) and open them in Internet Exploder 5 and show them how horrible the death and destruction is.
Then tell them you'll charge double to get that looking nice in IE5. (or whatever ancient browser you're going for)
IE6 is still viable in some markets so this is why corporate sites are simply not going to be using fancy CSS3 or newer markup for a while yet. The fancy designers, they make a living by looking pretty, so they aren't going to be coding for IE6 anyway unless they have to.
"but why doesn't it work on IE4 on a mac? I thought good developers considered 'IE support'
You have to be clear at that first meeting with the client what you support and what that means. Paul O'B I believe actually has or had a client who needed pages working in IE/Mac. They're still out there, like sparse survivors after the zombies took over the world.