Given the way we treat our cars today vs the way they were treated when new and just cars......in my opinion, it is not a big deal whether you paint those blind areas or not. I personally do not. Where the paint gets between the pan and intake, it gets. And where it doesn't, it doesn't. You can't see the difference when looking at them. And I haven't experienced any other negatives.

But, if a guy wants to go to the time and trouble to prep, mask, and squirt the valley pan and bottom of the intake, then good on him. Although I feel it is a bit overkill, other than the extra work, there is no other down side.

As an example; I have a couple of late 70s motor home motors in the barn. Complete and original. And the valley pans are intact. I have seen valley pans rusted through, but they were exposed to the elements for extended length of time.