Hi Amanda: The answer is (as with most things in life): it depends! In the case of my own dogs: never in a decade or more (well, one has been once -- because she was a rescue from the pound and her coat was so matted). Most cavalier puppies do not need to go to groomers because their coats take nearly a year to come in, and then they tend to lose most of it (blow it) around that age.
But some people might go the groomers for a wash and dry for a puppy, to get them used to seeing a groomer early on.
Cutting or shaving cavalier hair is pretty controversial. Most show breeders and experienced owners will say that once the hair is cut (which, under the breed standard, it actually isn't ever supposed to be), the coat will be damaged and grow back more curly and fluffy. For some people this doesn't matter to them and they like a shorter coat or find it easier to manage. Others find the look disturbing and wonder why people get a long haired breed known for its coat, only to shave it off! There are many fierce and sometimes angry debates over this (welcome to the world of passionate cavalier owners! :lol
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Most dogs actually need the longer coat as protection in both winter and summer. Some (mostly, mistakenly) think dogs stay cooler with it cut very short but this can expose skin to sunburn and risk melanomas, especially in the part-colours (the cavaliers with white coat, blenheims and tris). Just in contrast, it is always advised never to shave the coat of my other dog, Great Pyrnees (Pyrenean Mountain Dogs) even in summer as it provides insulation *against* heat.
That said, even show breeders will do a bit of evening up of the coat and it is fine under the show standard (and needed for comfort by most cavaliers) to have the hair between the pads on the undersides of the foot carefully trimmed out with a blunt scissors or by a groomer, so they don't slip on floors (but the long hair on the TOP of the feet, the 'slippers', should be left long or else just neatened up but not taken back like 'cats paws'.
For those who trim their own cavaliers -- please please invest in a small BLUNT (rounded tip) special dog scissors for doing any area around the face or feet. Don't risk a painful and potentially gruesome accident
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Unfortunately 95% of groomers (in the experience of forum members at least!) seem totally unaware of how to groom cavaliers and often, to the alarm and heartbreak of owners, return dogs totally shaved or severely trimmed. It will take up to a year (or longer) for coats to fully regrow.
All that said -- it's a personal decision. Many pet owners who keep coats long do opt for a 'sanitary trim' that trims up hair prone to getting peed on (and thus smelly). They may neaten up ears as they can get so long dogs step on them and they matt frequently. For dogs with really thick or cottony coats, they may have groomers use a thinning scissors to take weight or frizz off (or just buy a thinning scissors and do it yourself -- much better results than trying to cut hair with a regular scissors, for the non-expert!). Also: many cavaliers that have health issues like CM/SM find brushing and regular grooming too uncomfortable or painful, and keeping coats short can make their lives better -- or even just keeping ears short, as the scratching that is a common symptom can mat hair and cause potential distress too (eg the foot can get caught in the mat when scratching). There are lots of YouTube videos on grooming and trimming but look for those on cavaliers and by professionals, not people shaving their dog at home :yikes
If going to a groomer be absolutely specific and clear about what you do and more importantly DO NOT want done. Write this down for them, ideally. Cannot say this enough; as most groomers WILL trim dogs back severely, often like cocker spaniels, or shave them completely including ear hair
). If you just want a wash and dry make sure they know this!
A GOOD groomer is a real find. Good reasons to see a groomer: they do a great wash and dry and use products that will keep hair clean and untangled for far longer than a home wash generally will. They will do tricky areas like the paw pads. They will (and generally should) trim nails and also often will do anal glands. They often will notice potential health issues first (eg lumps that need checking, signs of pain).
That's a long answer but I hope takes care of some of the pros and cons around the issue!!