I use salt to adjust the texture (by reducing moisture) in the final product and as a flavour enhancer. I do not use salt as a preservative as the quantity required to give any meaningful preservative effect would be quite unpalatable.
My choice is for the salmon fillets to be 'finished products' prior to salting. Osmosis happens equally on both sides as does the smoking process.
How long I leave the salmon fillets in the salt is very much the 'black art' . The less fatty the fish, the more accurately I need to judge the timing.
Here's a wee list ....... Age of fish. Sexual maturity of fish. Stocking density of fish. Location of salmon cages ( tidal flow and water temperature) . Diet of fish. General health of fish. pH of fish (indicating stress related issues causing lactic acid build up which will be noticed on the palate by a less unctuous even minutely acid flavour and a much shorter flavour finish). And how long I age the fish for (even when the whole fish are kept permanently on ice I will age the fish for fewer days in the Summer than in Winter before I feel they are at their optimum for filleting and smoking).
With time and experience all of the above can be fairly accurately judged by the look and feel of the whole fish and then by the look and feel of the fillets .....
A note on salt. Salt dissolves at different rates depending on it's granular size but also on it's geographical origin, method of manufacture and also the ambient temperature and humidity at which it's being used . Import regulations have forced me to use 3 different salts this year. And each one has it's own characteristics. Out of preference I use kosher organic sea salt.