In the past we have treated sous vide cooking and salting similar to traditional methods, where you salt before the cooking process. However, some new findings are showing that at the long times and low temperatures involved in sous vide, the salting can actually make the meat less juicy.
The main reason salt makes meat less juicy is because it begins to cure the meat, denaturing the proteins, and drawing out moisture. This results in dry meat, one of the common complaints for long-cooked sous vide beef.
This is especially apparent in red meat that is cooked for a long amount of time (greater than 4 or 5 hours) or meat that is a part of the cook, chill, and hold process.
During our tests we've found mixed results. The no-salt version was definitely juicier, there was less liquid in the bag, and the liquid was more clear. However, the salted version seemed to have a beefier flavor. We did the tests on a chuck steak, tasted after cooking for 4, 24, and 48 hours. Chuck steaks are very juicy in general and we plan to try more tests with a leaner cut such as eye of round to see if the difference is more pronounced.
While some people do not like the more cured flavor of pre-salted meat, most people prefer fish that has been salted. This has a similar effect, with a firming up of the flesh, and often gives the fish a much desired bite to them.
To add the salt to your fish you have 2 options. The first option is to salt it as you normally would, then let it sit in the bag for 30 to 45 minutes (in the fridge). The second option is to make a brine and place the fish in it for at least 15 minutes and up to several hours.
At any rate, the decision to pre- or post-salt is one more variable to keep in mind as you cook sous vide meals. When cooking red meats for long periods of time or using the cook, chill, and hold process we now recommend not salting until after the meat comes out of the pouches and is ready for searing.
For more information about salting and sous vide check out the great posts by Ideas in Food Corrective Seasoning: Chasing Juicy and the French Culinary Institute: To Salt or Not To Salt? That's the Searing Question.