Yes, storing nuclear waste underground is a good idea, because it is the safest place to store nuclear waste. Relative to coal energy, nuclear waste is clean. While the waste has to go somewhere, putting it underground insulates it from causing problems later on. Scientists have developed a thorough system for storing the waste, and it is sufficient.
Storing nuclear waste in underground repositories is a very bad idea. While there may be no effect that show themselves immediatly, no one is able to know what will happen in the future. The risk of the effect of storing nuclear waste underground does not make storing it underground a good idea.