The way my group plays, we generally add up everything for a given action, then apply halving/rounding as necessary. The only exceptions we use are:
1) Capital Ship Squadrons, where we do it ship-by-ship then add it up (because cap squadrons tend to get complicated with different damage values and crippled statuses, etc) and
2) Firing weapons at different targets, so if a unit is firing a dorsal battery one way, and a port/st'bd battery a different way, we resolve each one separately. I don't know if it's come up, but we might have applied this principle to launch bays as well (so a crippled Styx would either be able to treat each bay as Str 3/2 = 1.5 -> 2, but wouldn't be able to combine them, or combine the two sets of bays to launch a Str 6/2 = 3 wave), however there isn't any explicit rules allowance for doing it that way.