I get blasted for this all the time, but the ONLY time you really need to worry about low humidity is in clone and seedling stages. As above the transpiration works and it helps the plant 'breathe' to move fluids from the roots to the leaves.

I live where my humidity over a year will be at 90% for a long period and then down in the 20% range for the rest of the year. I make no concessions or attempts to regulate the humidity other than VENTING the room to remove EXCESS moisture to prevent mold and fungus.

Mother Nature does not control the world's humidity well either. The stuff grows wild quite well in arid climates.