Also, a small amount of caulk on the underside of every screw head.
This is good practice, but it is even better to take it further.
Over time the putty can move and or shrink, or if installed on a cool day; compress/settle in hot weather. What can happen is that the trim can become loose. The screws stay as placed, but the trim is no longer as snug. When this happens, the screw head will still be sealed, but water can get under the trim and follow the screw threads into the wood.
The best solution is after installing everything, remove a couple screws at a time and inject sealant into the screw hole. Re-install the screws and both the threads, and screw head will new be sealed, and will stay sealed even if the trim becomes loose or the putty shrinks.
Rob