It depends on how much your bong typically evaporates. If it's huge, and evaporates water like there's no tomorrow, then:
A: it's going to be really humid in your room
B: it'll need refilling quite often.
Since I'm going to set up my bong tomorrow (Hopefully

) I'll find this out first hand. rivercom9, if you want me to PM you after a few days of it running, with how much it evaporates, I will.
It would be a good idea to used sitilled water. Using tap water, over time, will corrode your waterblock. If distilled is too expensive, then "spring water," "filtered water" etc etc would probably do.
Don't put water wetter or antifreeze in your bong, unless your want to smell them all day.