'As confirmed by many others', who clearly aren't running a boot camp partition by the sounds of it. Without that I'm using 212Gb of 512GB, and that's with all my media on an external array. Email cache, Lightroom previews, all the 'stuff' that the OS shoves on the system drive by default (such as application libraries). I could tidy up, a little, but I'd still be at over 100GB just for OS X and my apps. Add a Windows partition too (with all its apps) and you are close to the 256GB limit. It would fit, but it would be tight and I'd be spending time juggling things around.
Without the bootcamp partition I would agree 256GB would be enough, but with it? Get the 512GB, it's not a massive increase in cost but will save you problems in future when you run out of space and wish you would have gotten the bigger drive. Considering that Apple seal up the iMac so you can't upgrade I don't know why anyone would go for the 256GB, I'd even be tempted to say go for the 1TB and have done with it. Or is all this too expensive for you? ;o)
Holy frakk
My MBP '09 (had it for 6 years now) is still having 180 GB free space of its 256 GB SSD.. For me, even with Bootcamp, 256 GB would be enough. I guess those mail caches eat up a lot of space - I just use webmail version of Inbox.