I used to use 2.5 but found it left some empty space inside as I stitch with a 1/4" seam. I use 80/20 for batting so my quilts are not super thick. I changed to 2.25 folding the fabric in half for a double thickness and it still leaves a little space inside so may knock it down to 2.0 next time but will make that decision once I wash it as it may shrink just enough.
Of course as others will state, it also depends on how thick your quilt sandwich is as to whether you would use a larger binding or not. So far I haven't had to.