Rather hard to answer such a generic question with so little info. Higher voltage is ok, as long as you don't burn or dryhit your coil. And as a general rule, the more juice evaporates, the more gunk will form on your coil, aslo greatly depending on what kind of juice an flavors you use.
I can however imagine that long hits at lower voltages could cause your juice not to evaporate enough, making your coil flood or leak. But then you'd notice soon enough.