Here is an article talking about all the medical risks and benefits of S/N.
From all the research I've read on the issue, it seems it is beneficial (healthwise) to spay a female dog, but it's not so much of a health benefit to a male dog (though it can prevent unwanted habits like marking in the house).
If you don't want to have to take the effort and watchfulness it requires to keep your dog from getting pregnant or impregnating another dog, it's best to spay or neuter them. You have to be always watchful and in control of an unaltered dog, and most people do not want to have the extra worry and the chance of ending up with unwanted and unplanned-for puppies that will have a good chance of ending up in a shelter. It is hard work to properly care for a pregnant dog and raise her puppies to be well-adjusted family pets (the process starts BEFORE they are even 8 weeks old, which is when the overage owner gets a puppy and therefore doesn't know what should happen before then). For an amateur it is not a good idea to take the risk.