I personally go dual-skill SSR20 -> SR15 -> SSR10 -> SR20 -> SSR20

Do I have a mathematical reason for it? Not really no. But since each Assault level gives you the same benefit (except for dual-skills which gain double), that's probably fairly close to an optimal route. If we're not talking about as optimal as leveling everything exactly equally, which would certainly be better, but boy would it be a pain.