Congrats on rescuing a dog! My wife and I volunteer at our local shelter to walk dogs. It is right down the road from our church, so we do it every Sunday. Good to see that people are still rescuing.
As far as the insurance, I looked into it, as well. We have an 11yr old Boxer. At one time we had 2 boxers but the female died. We checked out the insurance and decided against it. The reasons were our reasons and I am not saying you shouldn't get the insurance, but we didn't for our own reasons.
Like someone earlier said, his/her payments are $33 a month with a $100 deductible. We get 6 months of heartworm pills for less than that. Other than getting those pills, we don't really need to take him (Pete, named after Pete Rose) to the vet. So, if you follow the Dave Ramsey plan and put money in an envelope each week for a designated purpose, I don't see the need for pet insurance.
You could put $33 a month in an envelope and use it only for vet bills. It would also help cover your deductible. You will probably find that your dog doesn't incur $33 a month in vet bills after a $100 deductible and therefore, save money. But keep that money in that envelope and keep adding to it once you decide what you are going to pay each month.
Like I said, its what I did and it has worked out well. Can't say it will work for everyone.