Aye, low humidity is best.. It helps, along with adequate air-flow (couple oscillating fans), to prevent mold, which also helps to avoid fungus gnats and multiple plant maladies such as fungal rot, etc. Around 40-60 is good, anything lower and you can do the wet towel trick (I do in the summer, and it works great), any higher, well, I dunno, you have got some problems. My house has brick walls, so it makes for weird humidity levels in my house (porous brick loves moisture). But, yeah, if you are at 40 or whatever, you don't have to be worried... Now, what is your grow space like? Does it have good ventilation, etc?