Yes, you can set the tub on the framing and then tile up to the tub rim. It's perfectly acceptable construction method. The reason I don't do it that way is that then the cut edge of the tile is showing, so you've got to be really good with your cuts. I'd rather have my cuts tucked under the lip of the tub. :-)

Ditto what weedy said. I helped DS/DIL tile a tub that was already in place. It took some tedious cuts and careful grouting to get a finish I was happy with between the tile and tub.
I tried caulk and it just didn't give the smooth look I was after.....plus, we couldn't find it in the right color.