Your dimensions should definitely give you a nice, roomy fit on the quilt.
The taper is a common feature on quilts, but not present on all. The first open ended quilt I ever really took note of was from Jacks R Better, and was a rectangle like you are planning. I can't say I have used a rectangular quilt, but I think it should work fine.
The reason the taper is common may just be part of the UL mentality. If the quilt doesn't need that extra space, why should it be there?
As for the amount of weight you would save, that depends on how aggressive the taper is. I just checked on SketchUp, and think that using those fabrics, my taper would only save about .75oz. So you are right in deciding that the difference is really not very significant.