Rough framing for a bathtub

Does anybody know how to frame for a bathtub (built-in on three sides)? If it is a 5 foot do I frame the walls 5 ft apart so that the flange attatches to the studs and the wall board then slides over the flange? Or do I fame it 5' 1" and put the wall board up before I install the tub?

The rough opening is measured stud to stud - without the d-wall. The The flange attaches directly to the studs. The drywall goes over the lip of the flange and the tile goes on top of that.

The walls are framed at 60-1/4" so that the tub can fit in there.
Or you can say five feet plus one quarter inch.

The backer board fits inside, not quite touching the tub.

What he said, and I'd add to make sure that you check the actual dimensions of the tub you plan to buy (most company websites have .pdf files with the info). Sometimes 5' doesn't really mean exactly 5'. ie my shower a 4' shower required a 47.75" opening.
OH

IF this tub is also going to serve as a shower, don't use drywall - use cement board (cbu). Assuming it is going to be tiled, don't run the cbu over the flange or it will bow out making it tough to make the tile look good - depending on the size of the tile, as long as they are big enough so most of it is supported by the cbu, you can stop the cbu at the top of the flange. If you wish, or if the tile is small, then either notch the studs so the thickness of the flange is even with the stud above the flange, or shim the studs so you can run the cbu down to about 1/4" from the tub surface. Also, put up a vapor barrier behind the cbu and run it down over the flange to protect the studs from moisture damage. If you are going to tile, check out www.johnbridge.com.

the rough opening for the tub (stud to stud) should be 5'-1/4".