The only time I've seen the input buffer drop below 100% significantly on either my home or work system was when doing seriously disk intensive stuff. And even then, no coasters. CPU intensive stuff doesn't seem to bother it at all. I always burn straight to disk, (ie. not creating an iso on disk first).
Work: 1GHz PIII, 512MB, IDE Software RAID1, Linux 16x IDE burner
Home: 1.2GHz Athlon, 384MB, 3Ware IDE RAID1 Linux 12x IDE burner