You need the time in the finish control box for validation purposes, but the "this Xkm ride was completed in Yhrs Zmins" box on the back of the card only needs to be filled in for BRM rides.
For the two shorter distances on my event, as they're BPs and I was doing instant validation at the finish, I didn't bother filling in that box - too time consuming - that's something riders can do for themselves if they like. For the 200, I had to fill in the times on the results sheet anyway so it wasn't much extra work to write the times on the cards.