It can definitely damage your gpu especially in the long run. So it's a good idea not to leave it at (let's say +100mv) on a daily basis. As that will degrade the life of the gpu overtime. (Though i doubt you'd have a gpu for 7-8 years now a days) I usually only apply an overclock when I'm about to play an intense game, then I apply the settings where I've increased the voltage. Generally though +100mv is safe. It's the sapphire version of trixx you have to worry about, because that program let's you go all the way up to +200mv. Which is pretty insane.
So to answer the question, yes it's safe, but will degrade the life of the gpu overtime. To counteract this, only apply these settings when you need it, i.e. when you're about to play a game or do some benchmarking.