Existing evidence shows that culturing embryos till stage 5-6 of development, we can increase the pregnancy success rates per fresh and frozen embryo transfer. This is due to the fact that the embryos to transfer or cryopreserve can be selected more properly, since blastocysts have a higher implantation potential. It also improves embryo-endometrial synchrony.
One should keep in mind that about 50 percent of viable embryos will arrest on day 3 of development. In other words, they will not make it to blastocyst stage.