Here's my story and why I suggest making reservations, at least on your way back.
We didn't make reservations: I went with my friend, we got there early, going to Vancouver Island was easy enough, got right on. On the way back, we knew what the schedule was. About an hour prior to the second to last ferry was leaving, we headed to the port. Well, there was a VERY long line. Long story very short. We found out that the second to the last ferry was full and on their way to Vancouver. the LAST ferry....well, the ferry ppl started counting out cars and asking anyone if they had reservations. Those that did, moved out of the line and up. Those that didn't, sat there. They started counting out cars, we made the last ferry, by about 15 cars. We were panicking because we were flying home the next day. No ferry, meant staying overnight on Vancouver Island and missing our flight. We started wondering if we could make any of the ferries elsewhere even one to WA. We started to call the airlines to see what we could do there too.
Overall. We BARELY made the last ferry. Everyone behind us, was turned away. They had to figure out what to do. So, my two cents. If you're going to make reservations, do it on your way back. Because if getting off the island is important, MAKE SURE YOU CAN LEAVE.
I wouldn't ever make reservations going, but will always do it after my stressful experience of going back to Vancouver and almost not making it.
JMHO.