As Katya said,
every tank can leak. As a dog owner (see my avatar

), I will also say that any dog can get it into their head that chewing up your PV is the right thing to do and can possibly do that any time they have free access to it.
I generally vape zero nic juice or when I use a bit it's very little so I'm not up on this stuff. I just Googled and
found the following at a couple of web sites:
"The toxic dose for nicotine in pets is 1/2-1 mg per pound of pet body weight, while the lethal dose is 4 mg per pound of pet body weight".
So if you use 18 mg/ml juice and have a 3 ml tank, you have 54 mg of nic in a full tank. By the above data that could cause a 108 lb dog to have some symptoms and it would kill a 13.5 lb dog. I have no idea how accurate this is.
If a healthy, non-inebriated person ingested too much nicotine they would become sick, vomit up most of it and recover quite well. I think dogs are the same, but again, I'm not sure of that.
If you're worried about it I think the best solution is to keep your tank small enough so your dog could eat it all and be OK. This still of course leaves your juice bottles as a risk that you'll have to get a handle on. Of course nicotine is not the only thing we have to worry about or dogs getting into so however you handle the rest should work for this too.