I always say that one way of thinking about this is: how willing are you to risk having them stolen or eat something dangerous? That's kind of the bottom line from the safety point of view. The broader issue is that of welfare -- dogs left outside on their own tend to become problem dogs. It's boring, first off, and not very comfortable, and out of boredom dogs tend to bark (what else is there to do), dig under fences, climb walls (cavaliers have been known to scale 5 foot walls without any problems!), chew what they shouldn't. There's a post stickied in the training section that is worth reading, from a trainer talking a bout why dogs really cannot be left outside all day. That can explain things in better detail.
I will say from doing rescue for many years, that the most poorly socialised dogs with the most problem behaviours and poor housetraining, are those that have been 'back garden dogs'. In a regular working week, dogs left outside will spend the majority of their lives sitting alone outside.
Cavaliers are one of the most stolen breeds internationally. They are easy to grab, easy to sell, and attractive as dog fight bait (little resistence to 'blood' new fighers
) and to sell on to puppy farmers who will keep them in hideous conditions. This is a growing area of criminal activity as so many people work all day and leave their dogs in the garden -- easy to grab even if you have to climb over a few gardens. We are having a huge number of thefts at the moment in Ireland, north and south (and have had recent public warnings from the SPCAs and police forces to keep dogs inside during the day, never tie them out at shops, never leave in the car even for a moment), and you see regular listings of stolen dogs too in the UK and US. Don't risk it.
As noted it's really good to be asking this up front.
That's what makes for a great dog owner!