On coats: I think coat is still highly dependent on genes both unneutered and neutered. Of my four, all spayed and neutered, only one has a cottony coat but she also has mild dermatitis on her back, where her coat is cottony. The black hair is cottony; the white isn't. Go figure!
Jaspar was neutered at 8 months, Leo at 10 months, Lucy at 6.5 years.
On females --
* a female cavalier spayed BEFORE her first season will run no risk of mammary cancer
* a female cavalier spayed after her FIRST heat runs a 7% risk of this cancer over her lifetime
* a female cavalier spayed after her SECOND heat runs a 25% risk of this cancer over her lifetime
That's probably the most common cancer for females. The other reason to spay is of course, pyometra.
According to a Swedish study, female cavaliers have one of the highest risks of studied breeds for the serious and sometimes fatal infection pyometra -- a huge 41% risk over their lifetime (the average across all breeds was 23-24% risk by age 10). That is extraordinarily high for cavaliers -- nearly double the average. The ACKCSC breed health survey for cavaliers a few years ago noted females had a shorter life expectancy than males and I wonder if the pyrometra fatality rate might have been a factor.
http://apt.allenpress.com/perlserv/.../0891-6640(2001)015[0530:BROPII]2.3.CO;2&ct=1
Some like to wait to spay til around 12 months -- in that case, just be aware of what is involved with going through a heat (eg keeping the dog inside for 4 weeks, no walks, etc).