[Ed Tipton]

Grant...I have found that 5160 can be "tempermental". Generally speaking, there are three things that lead to cracked blades in 5160.
I have not found that it is necessary to count the number of blows to each side. I merely look at the forging, and try to assure that it has a balanced and equal appearance. If one side receives a few more blows than the other side I don't think it really matters all that much.
Quenching the blade in oil will improve your chances, as will removing all stress risers, and avoiding working the steel when it is too cold. These three things are all under the smith's control.
Removing the stress risers is easily done, and should take only a minimum of time. Ensuring that you only work the steel at the proper temp is simply a matter of discipline. If these two things are done, the quenching medium can sometimes be water, and almost any non-flashing oil can be used. Having said all of that, cracks can and do happen even when all precautions have been taken into account.
As a side note, warping is also reduced by the same metheods...especially considering that both a crack and a warp are really nothing more than the steel attempting to relieve it's internal stresses.
it's also worth noting that even a coarse grade of abrasive can create stress risers. Any abrasive of 80 grit or coarser can cause stress risers that can ultimately lead to a crack. Another possible cause is forging the edge too thin prior to the quench. A thickness of about .030 (think "dime") is about as thin as you should go. Leaving it thicker does no harm, but it is just that much more to be ground away after the blade has been hardened. Forging too thin at the edge in effect creates one long stress riser along the entire edge.
It's a good bet that the quench is probably going to be the most stressful event in the entire life of the blade, so it behooves us to remove as much stress as possible to give ourselves the best possible chance of success.
Another thing that helps is to normalize the blade prior to quenching, and also to thermally cycle the blade to reduce the grain as much as possible.
After forging and grinding the blade, there are many stresses that have been introduced into the steel. All of the above mentioned things are done in an effort to reduce to the greatest extent possible the stresses that we have put into the steel leading up to the heat treatment.
The end result is an indicator of how well you have understood and controlled your process from start to finish. The entire process is done under the smith, but if the steel itself is questionable, then even the tightest controls may not help. This is why we shy away fron using "mystery metal" as much as possible. An experienced smith who has good control of his process may be able to work mystery metal to a satisfactory end...but for the less experienced, there are many pitfalls. It is always advisable to work with a known steel whenever possible.
In today's world, there are so many different types of steels available to us. Deciding on a steel that is best suited to our needs is something that needs to be done early on. Continuing to work with one steel until you have developed a good working relationship with it will pay off big time when you encounter the inevitable variables in steel. You've already discovered that all 5160 is not created equal....some is better than others. This is true of all steels, and is a good reason for us to "dance with what brung us". As we improve our process, there will be plenty of opportunities to spread our wings and venture out into the vast unknown...but your basic knowledge of working with your chosen steel will always be there to call upon as needed...and it will serve you well.