Butter and eggs cause heart disease too. (don't worry about switching to margerine, it causes cancer and heart disease, and it doesn't taste as good to boot)
The amount of sodium in softened water is less than what you habitually shake onto your food before you even taste it.
A saltine has more salt.
In fact, I believe I read recently that the American Heart Association released newer findings that said salt intake did not directly affect heart disease. (next year they'll say it does, but then if they really can't tell, it couldn't affect it much.)
For all of the things in the world worth worrying about, this one is pretty far down my list. I worry a lot more about my 7 & 9 year olds playing at a friend's house and finding their parent's handgun. (gee did I have to bring THAT up?)
You can buy inline water filters for your kitchen sink, or your refrigerator if you are lucky enough to have a dispenser, (I love ours!) In fact, our GE refrigerator has a Culligan water filter built into it.
You could also buy a whole house water filtration system, or if you're building, you can create a partially filtered system where the filtered water only goes to the faucets and shower, while the unfiltered water goes to toilets, washing machine and outside faucets.