I do it every night.
It must be OK logically - I imagine the most common time to charge a phone is overnight - if you're out all day, or work where there's no option to charge, charging whilst you kip is what has to happen really.
Over time, batteries do lose a little of their power, but I doubt leaving the phone plugged in once it's hit 100% is doing it any harm. I've never found that to be so, at least not so I've noticed. In fact, leaving it to 'over-charge', so to speak, gives me more battery (I have an LG G3). If it take it off charge after sleeping, i can often use it for ages before it drops to 99%... but if I take it off charge as soon as it hits 100%, it goes down immediately and continues at the same rate throughout the day.