Actually you can do it either way. When I worked for AAA, I would always offer the service, but sometimes clients wanted to do it on their own. That was no problem! The only time I would not offer it is if we were about a month out and they were going in the busy season. It is so difficult to get some restaurants (Le Cellier sells out EVERY night! as does the castle)
So, as long as you aren't going into your reservation itself (which you don't for dining reservations) you can feel free to do it on your own.