<p>I've read a lot about the pros and cons of sizing with either relative or absolute font sizes. Fixed sizes don't zoom in IE6 but that's not much of an issue these days. Accessibility is important, but I assume that all good accessibility software is built to deal with these issues?</p>

<p>I guess it mainly comes down to whether you want to be able to change all font sizes with one rule (i.e. the default font size you set) or whether you want to be able to change a font size somewhere without affecting nested elements (this is the thing that frustrates me most!).</p>

<p>Anyone have any tips?</p>

The only big reason valid is that a lot of people is still using IE6. Just this.
Probably could be a good thing, at the end of 2008, to give no more attention at IE6. But is difficult to ignore IE6. I work as web designer/devolper for a big company in Italy, and many of ours employes  or clients, in their work computers, are browsing the internet with IE6 right now.
Why? They don't have administrator rights and can't install anithing in their machines, and the system admistrators are always too busy to upgrade the computers of every office. May be this people will upgrade to IE7 (something like  Firefox is a dream) when their hard disks will need to be changed.
Is possible that this sort of things happens only in Italy. But I would not bet on this.