It's up to you really, just be aware that if you are doing vinyl, it may be helpful to lay out the job with an eye toward how the last course at the top of the job will end up. It could save you from having an odd sliver of material at the top
I like to keep it about 3/4"-1" below the bottom of the sill. And that is at your lowest corner in case one corner may be a little lower than others. By that I mean that you first want to check your level all the way around, then start at your lowest point.
