Sealing a deck - exposed but unaccessible surfaces?

When one seals a deck, you normally want to apply sealer to all surfaces.
What happens to surfaces that you can't reach, like those that abut a brick wall, or the sides of planks that sit right next to each other?
Should I be crawling underneath the deck and sealing those surfaces as well?
Any advice on this dumb question appreciated!
- Daniel
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