I've found that ies4osx doesn't reproduce the behaviour of IE browsers closely enough - always a corner case here or there. If you can get VMware and a copy of Windows or as the OP mentioned, Parallels, it's the more desirable way to test IE.

Ah yes, I'd forgotten about that - I have installed Macports, but find ies4osx a little slow/crappy (compared to Parallels), but it was definitely handy before I splashed out on parallels.