I don't think it matters. I did some testing years ago on strength of end grain glue joints. I used straight glue as well as glue mixed 50/50. No difference that I could tell. What I found out from my tests was if you want any strength at all use epoxy. I tried a bunch of glues. I glued 3/4" square 6" long pieces together. I clamped one end in a vice and then put a 2 lb weight on the other end that had a long stick clamped to it. I would slide the weight out until the joint broke. All the glue except yellow glue where I sized the joint with glue before applying the full strength glue and the epoxy broke before the weight even go to the end of the 6". The Yellow and white glue held up to about 7 or 8 ". The epoxy held up to about a foot and a half. Polyeurethane glue surprised me by being extremely weak. Actually I was surprised at how weak all of the glues were in this test. It proves that if you need to glue end grain to end grain you to provide some sort of mechanical edge or brick layed long grain like they do in segmented turning if you want any strength at all. I will be doing a test this morning on a scarf joint strength. This has a lot of end grain and side grain in the slice. I want to see how it breaks. My thoughts are it's far stronger than end grain to end grain but probably not as strong as side grain to side grain. We will see.