As Jane says, I think it is definitely wise to book. We ate in El Posito twice during our recent holiday. On the first occasion we had made a reservation and everything went perfectly however when we arrived the following week without a reservation we had quite a long wait as the restaurant was very busy. Entirely our own fault.
We also ate in Ca N'Olesa twice and did exactly as Jane recommends - booked a table, went for a pre dinner drink and stroll and returned to find our table ready. When we left we booked a table for the following week and again arrived to find it ready for us. Given the number of people standing around waiting for tables I think it's a much better idea to book at this time of year rather than be disappointed.
LK