Just to clarify: longtail (aka double) caston makes an edge that looks like it has already had a row of knitting done on it. It's in the shape of the caston stitches, that look like the bumps you get on the back of a row of knitting.
But it is not as long as an actual 'caston, knit one row' would be, It's not a knitted row in my opinion. BUT in stocking stitch, I recommend that you make the first row you work a purl row, because that side of the caston already looks like it contains a purl row so you should make it that side.
However in actual practice if you end up doing one row fewer than the pattern calls for it is unlikely to make any difference, so don't worry about it much. Also, if you find that you actually like the look of that side of the caston, you might choose to put it on the right side of the work.
So for longtail/double caston, some people say it's a knitted row, but I think all that really means is that they THINK it looks like a kntted row has already been done.
For all other castons, there's no debate: caston row is not the first knitted row, you cast on then knit row 1.