On the basis of your question, as it is, I would say that there are many more things to worry more about than kidney infection. With a mechanical valve and presumably taking anticoagulant, I would be much more concerned with preventing head injuries than kidney infections. Are you at particular risk of kidney infection? Do you have some kind of condition that would make it worse for you than normal?
It depends, but usually urination helps prevent kidney problems. If you have any urinary problems (e.g. in men an enlarged prostate) then address them.