I'll preface this comment with the caveat that the only time I took the Cedar Island ferry was in May 1993, so my experience may be of limited value.
With that said, I would make a reservation for that particular ferry. The ride is long (over two hours) and the boat is relatively small compared to ferries you may have ridden elsewhere. It's better not to take the chance on it being full because then you'll be stuck either waiting for a later sailing or having to go back and drive the long way around to get back to the Outer Banks via the land route. Cedar Island is out of the way enough that it simply doesn't make sense to take the risk.
For the Ocracoke-to-Hatteras ferry, however, you don't need a reservation. It's first-come first-served. The road up Ocracoke Island ends in sort of a teardrop shape and you line up along the teardrop. Depending on your tolerance for risking a speeding ticket, once you get out of the town of Ocracoke you might want to drive fast enough to try to beat the rest of the traffic coming from the Cedar Island ferry if you don't want to wait on line for a long time or possibly have to wait for the second ferry to Hatteras (if memory serves, the Hatteras ferries are somewhat smaller than the Cedar Island and Swan Quarter ferries, in part because they run more often–but I might be mistaken on that because it's been so many years).
When navigating for my parents on the road in the past (they're awful at following directions on their own), I will admit, I've lied about directions and said there's traffic or the road's closed or what have you, in order to clinch a route or see an interesting road-related site. Either that or I'd tell them nothing and add my own stops on the route. The most extreme I'd ever taken it was when we were going to Gettysburg PA from Fairfax County VA and I routed my mother over White's Ferry on the Potomac. That added an hour and a half to the trip. Had she known this detour was totally unnecessary she probably would've thrown a fit.
Heh. Back in August of 1990, we were headed towards the White Mountains of New Hampshire on a family vacation. My dad got tired and asked me to drive because my mom didn't feel like driving. The two of them then dozed off. My brother and I decided that it wouldn't be too far out of the way if we detoured to Maine, specifically Freeport, to stop at LL Bean (at the time, the Freeport store was their only store). Dad was mildly surprised when he woke up to find we were on the Maine Turnpike, but he didn't mind too much because he liked visiting that store. The year before, when we stopped there en route to PEI on a Boy Scout trip to the Canadian Jamboree, we were there for several hours and only left when I pointed out that it was still several hundred miles to Moncton, where we had motel reservations that night.