All else being equal, it is far more of a health issue to get your female neutered than your males, but if you don't neuter the males and do neuter your female, you are almost certain to start getting leg lifting etc in the house and humping behaviour and other things you are not seeing now. If you are going to do the boys it would be a good idea to neuter before you add a female to the house so they don't begin and learn behaviour you do not want. It can be quite hard to stop once it begins.
There is a whole post on reasons why to neuter males and females in the Library section, but one of the key ones for female cavaliers is that they are one of the highest risk breeds for the often fatal womb disorder called pyometra. Often this is not spotted by the owner until either too late to save the dog or an emergency and risky spay is the only solution -- and typically costs can go well over $1000-2000 for an emergency procedure of this sort to try to pull the dog through.
If you don't neuter any of them, you have a major management job on hand which even most breeders do not undertake -- most will keep typically keep girls but not intact boys and girls.
This breed is very prone to weight gain anyway because people overfeed and underexercise, especially as they get older. So as others say all this is really up to the owner to manage and isn't difficult
. Many neutered dogs need 10-20% fewer calories -- meaning less food and a cost savings.
Coat change can be genetic or connected to diet, weight etc though some may change with neutering -- but really, this is a minor cosmetic issue that 99% of people will not even notice. Of my 5, only one has a cottony coat and it is likely a combination of reasons that she has this. Ad funnily enough people often remark on her coat in particular in a positive way as they say she is so soft and fluffy! So much that the owner might think is an issue is actually a matter of total indifference to the rest of the world. :lol:
Many coats change as the dogs get older anyway -- I know lots of intact dogs with dry, cottony, dull coats, or coats that fade or go curly. Good healthy diet and coat care and exercise are really what matters to maintaining an attractive coat.