Being honest i always try to survey a site. 1. Because it is always good to be professional and meet your possible client, 2. If you dont and then get the job and turn up its sods law that you will hit a problem which ends up costing you more time and money, obviously on the bigger jobs more than small domestics.
better to be safe than sorry plus whilst on site you can ensure there getting whst they need rather than what they think they need from reading posts on google !