Is there a benefit to breathing every 5th stroke instead of every 3rd?
Yes, it's faster and it builds your lungs. If you are a pretty good swimmer looking to get better, breathing drills and switching breathing is a good way.
But I would only worry about it if you are pretty good already. Breathing every 3rd is fine. If you breath every 2nd stroke, switching to at least every 3rd is probably a good idea.
Yes, it's definitely good to practice hypoxic breathing every 3, 5 or 7, but only in the lap lane. As Lt. Mudd said it definitely builds your lungs, just like swimming underwater does. Breathing every 3 in competition is good because you can site on both sides. Every 5+ is not such, especially if you are really pushing yourself during the swim.
I've been wondering about this a lot over the past year. I've read many times that endurance training does not significantly affect lung capacity, that the limiting factors have more to do with heart pumping capacity, capillary density, neuromuscular coordination, bone/ligament/tendon strength and durability, and so on.
I still don't understand the goal of hypoxic training. For an endurance sport, you need lots and lots of oxygen. I'm just not seeing why learning how to swim hypoxic is beneficial.
Also, during the television coverage of the 2008 Summer Olympic Games, I kept an eye on the breathing patterns of some of the swimmers. I noticed that almost every single swimmer I tracked would only breathe on one side during races. Of course I'm sure they do plenty of alternate breathing in training so that they don't develop muscle imbalances of the neck and back. But most people have a stronger more efficient side so it seems like they fall back on that in racing.
Open water swimming is different because you face the challenges of having waves or other swimmers hitting you in the face on one side, so you have to be ready to switch. But even then, it should still be possible to breathe to your dominant side most of the time and switch as necessary to avoid the waves or flailing limbs of other swimmers.
I'm still relatively new to open water swimming and triathlon but I have been trying to look at examples of what experienced triathletes do.
PTri2009, you have some good questions. I saw the same thing in the Olympics with the breathing as that it was very frequent. Sometimes the swimmers breath to both sides to keep an eye on the competition. What you should try and observe is the Open Water 10k swim and see how they breathe. I didn't catch those races so I can't say for sure.
What hypoxic breathing does is it teaches one how to control ones breathing while swimming. It can aid you during you flip turns, or when you are in an open water event and there is a chance someone next to you creates waves so you can't breathe to that side.
There are various articles online that you can research under 'hypoxic breathing/swimming'. Try some of what you read out in practice to see if it's works for you.