If you stay off property, you'll need to pay to park at the parks ($14/day I believe). Keep your receipt and you only have to pay once per day regardless of how many parks you go to.
You do not have to pay to park at resorts, unless you want to Valet ($15/parking I believe). However, if you don't Valet, you're generally limited to 2 (maybe 3) hours "free" parking for visiting the resort/eating a meal. Anything over that and you are subject to being towed (and incurring the wrath of many here on the Dis).
True, if you plan on taking more than one trip in a year. If you'll only be taking one trip within the year; then it would still be cheaper to buy separate tickets and pay for parking.