I've had seeds crack in less time than 8 hours. Usually mine open within 24 hours, but I've heard of it taking over 48. For me, if they haven't cracked in 2 days I'd give up on them.
Just recently germed for 2nd time, I've heard of going straight to soil, plopping in other mediums, submerging in cup of water, or paper towels. I choose the towel method being that everyone who uses it highly recommends it and has optimal germ %s.
*You have the opportunity to SEE the taproot coming out and place it downwards with some tweezers for a head start,
*also you don't use up a medium such as a cube or soil for a dud seed.
*If you are using rockwool cubes, they need to be soaked for a day anyhow, which you can do while you're germing seeds, saving some time
Just wet towels, fold over, keep the towels damp to touch and place between a bowl and plate to keep in moisture and keep light out. Keep warm, and seeds will crack within 48 hrs. My recent batch took nearly the full 48 hrs.
The only possible downside I can see would be that some feel that sprouting straight into soil would be possibly a day or two quicker due to the sproutling starting off in it's new home for the next month (you WILL probably transplant again as most people progress the size of their planting pot with the size of the plant, transplanting at least once ~soil) If you're using hydro then there are many mini hydro methods for germing/cloning). Transplanting will shock the plant for a day or two.
Hope that helps your consideration!
~V~