Oh yeah - 40% - 60% relative humidity is ideal for acoustic guitars. Higher RH will cause expansion of the wood which will raise the action, deaden the sound slightly and a few other minor things. Very little risk of damage though. My guitars have spent many summers in 80% - 90% RH with very little lasting effect. At the other end is where damage comes from. 20% RH for any extended period of time is a guitar killer.

In reality, every acoustic guitar owner should own at least one digital hygrometer (the needle type aren't as accurate). $10-$20 gets you a lot of information that's paramount to your guitars health.