I've also been using grade 5 and 0. One thing that I learnt from the forums at APUG is correctly exposing the second grade (e.g. grade 5) test strip. When I exposed the grade 5 test strip on top of the grade 0 I was struggling to see big differences between the times. It was pointed out that I should be covering the paper in increments instead of uncovering the paper in increments. Voila! noticeable variations in the test strip.
The major advantage of using split grade for me is being able to burn in at a specific grade.