Yeah, Martin says that, never use anything, except maybe water. I use Petros Fingerboard oil, which you can get from Maury's Music. Just a drop or two on a paper towel, then just the areas between the metal frets as wiped on, then push it up to the frets as you spread it out. If the frets get oil soaked, then when you need a refret the luthier may not be able to get them to glue in too well because the residual oil prevents adhesion. Probably part of the reason Martin advises against any treatments. But a little oil really makes the fingerboards look nice. You use a little 0000 steel wool first to clean em up, then wipe away the iron filing with a throw-away rag so you don't buff polish your guitar accidentally with metal shavings on your polishing cloth.
With regard to changing strings one by one, if your guitar ever "goes into the shop," they're going to take all the strings off and they don't even think twice about it. However, I will say that every time I've had my guitars worked on they always come back to me with more damage done by the shop than I ever imposed. But they get you out of over protecting your guitar which means that you might let someone else play it, and you stop obsessing over perfection.
Keep your guitar humidified. It's winter most places. Very dry. You'll get a crack in January if you don't humidify at all. Worst month by far.