To be honest day 5, 6 or 7 doesn't matter if they went on to become blastocsyt...in fact it has to be stronger embryos to make it to day-6/7 in an external environment. Mine, I had 2 day-5, 3 day-6 and 2 day-7 blastocysts.
After doing PGS on the first 5, the euploid ones were of day-6. In fact my first day-5 blastocyst was quite high mosaic for various karyotypes.