You're going to pay a lot more for Apple drives than buying afterwards. However, if you ever want an SSD it's best to Apple order upfront because they use a different case to fit it. I also wouldn't be eager to open up an iMac case unless I was very confident and kept things very clean.
So it depends on how much space you really need. I keep all my files/videos/music on external storage which is shared on my network. So my actual computers use very little space for applications. 256 GB SSD in that case would be a huge amount of space. If you grow beyond that you could buy external storage for a much better price.
Another thing is, yes, SSD are fast but depending on what you're doing it may not be that big a difference vs the cost. Does having your boot up 10 secs faster really that important? If you're playing games or music or browsing, there isn't much difference. If you avoid the sata drive completely, you end up with a quieter and cooler iMac. Again, comes down to how important this is to you vs the cost. With all in ones, you're going to have to make these choices up front so imho it's always better to err on more than less.