It's tedious! If you can try doing it once you've finished an area, makes it less tedious. Saving it all for last, for me anyway, kills me. Like the others said it does make the picture more enhanced.
I think they should always be done last because your new cross stitches may overlap the backstitches. At least mine do sometimes. I always do the backstitching last.
Any more I now backstitch as I go, once I have an area complete with cross stitches I will back stitch the area. Too many projects have way too much back stitch and to leave it to the end I almost lose the will to finish the darned project. So, if there is a lot of backstitching I will do it as I go, if there is a little, I will do it in the end. Some designers seem to think every cross stitch needs backstitching and that is so not so!