Cavaliers are not supposed to be trimmed, and groomers should know this. Few in Ireland seem to though, and are very prone to shaving down dogs *as the norm* :yikes and also taking all the hair off their feet and trimming their ears back to puppy length -- all of which I simply detest (apologies to those who like this look but to me, they just look like beagles after all that, not cavaliers; the glory of a cavalier to me is its beuatiful coat, long ears, and funny hairy feet!
). So if you go to a groomer and DON'T want them to do any of these things I suggest putting that in writing when you bring the dog in and INSIST you want a shampoo and brush out but NO TRIMMING or at most, maybe thinning the coat somewhat in a heavy-coated dog or for summer. NB even though I made it very clear I did not want Lily's feet trimmed the groomers in Dublin did it anyway when I had her coat neatened up -- it was such a mess that I felt it needed some shaping. I was so annoyed; it took months for them to start to grow back.
Groomers seem to think this is what they are supposed to do with cavaliers; I guess a lot of people prefer to have them trimmed and I definitely see MOSTLY shaved cavaliers.
Cavaliers will not overheat in a typical summer in Ireland and the UK -- they are native to this climate, have a single not double coat, and if it is a particularly warm day just make sure they have shade and bring water on long walks or days out. But honestly -- we had a Great Pyreness in the Calif central valley with heat regularly over 100 degrees in summer and we never considered or needed or were advised by the vet to consider trimming her coat! A Pyr DOES have a heavy double coat, too. If a pyr can manage very hot summers a cavalier can easily be comfortable in its NATIVE climate in its Natural coat.
So no one should ever feel they need to shave coats to make a dog cooler or feel pressured by a groomer to do this. Keep in mind too that shaving a coat can expose the dog to excessive sunlight especially those with white in their coat as this is especially prone to sunburn and more at risk for skin cancer. A coat can also keep a dog cooler by protecting it from direct heat -- in countries that are very hot, the natives often wear long loose clothing for this reason.
The groomer I spoke to said they would charge less for a basic groom (eg no cutting) but still it isn't a priority for me to pay for three dogs to visit the groomers so I don't bother, I just wash them myself (they swim regularlywhich keeps them pretty clean). My groomer suggested every three-four months for a groom BTW. Every two months will obviously keep them looking nicer; two months is also probably more the norm for dog breeds that get trimmed.
Ears do need weekly attention or they can matt really badly -- armpits and behind the ears are my worst spots. Buying a good matt breaker and then using a pin brush designed for a cat and a grooming spray really helps keep matts at bay.