I've read 35% +/- is a good level for humidity, but should I keep it there throughout the life of the plant? Should the levels change from seedling to veg to flower?
What you read to was correct for a flowering plant several weeks from harvesting. As a flowering plant gets closer to finishing the RH should slowly get to 20% or even lower by the time you harvest. Seedlings like 80-90% RH and plants in veg should never have the RH lower than 50%,60% is best not to go lower than. 40% is good for plants going into flower and lower as it matures. Problems can occur in veg. if RH is too low just like in flower if the RH is too high. Temps and RH have a close relationship,spider mites and mold are also just two problems which i have recently encountered and are dreadful indeed.
in my experience, humidity is not too big of a deal. dont break your back to buy a humidifier and such if your plants are doing well in 30 % humidity. but in flower, humidity is actually quite important. High humidity (30-40% or more) in flower can definetly lead to mold problems and can ruin a harvest. EDIT: higher humidity in veg. sorta prevents spider mites,etc from taking over but ive never had problems with bugs in my house so.