I wouldn't count on them holding your ressie, it probably depends on the host/hostess at whatever restaurant.
We learned the hard way what happens if you are late this past saturday night. Had a 6:45pm reservation at Raglan Road, party of 7. All were there but two, I told the 5 to check in at 6:35 and ask to be seated, we were pulling up and parking. The hostess would not seat them, we arrived, 6:45 on the button, checked in again, with the other 5 who were there already and on time. They didn't even have us on the list..now keep in mind we were on time, not even late..ugh. Anyway, the hostess and crossed our name off the reservation list when the first party tried to check in, so we had to wait for a table and with a party of 7 it isn't quite as easy as a party of 2 or 4.
She said well mam it is only a priority seating, I said I know all about priority seating, but you gave our table away and we aren't even late, right..she said yes mam.
We got seated about 20 minutes later. The food was good, service was good, but the hostess on duty saturday night..chubby girl with black hair..not impressive at all and needs to be fired.