All Kindles starting from the Kindle 3, also known as the Kindle Keyboard, are now on KF8.
To put it another way, the only ones that don't support it are the Kindle 1, Kindle 2 and Kindle DX. Please correct me if I'm wrong.
There are not many of those old devices, so I would say that it makes sense to make everything KF8 if you can. You might start with only using a few small features, but over time consumers will expect more.
It is well worth downloading the official KF8 sample file and looking at that on your Kindle. Click on the bottom of the right hand column of this useful page: