especially in this economy, i think it's a bad idea to not reserve tables for walk-ins. Osteria, Garibaldi's and Sociale (all in the pretentious and average quality Laurel/Presidio heights area) all refused to give our party of 4 a table on saturday night. they either refused outright (sociale) or told us maybe we could eat an hour or two later. my reaction is to not support these restaurants when they're short on customers.
there are plenty of top 20 (or 50...) restaurants in SF that take walk-ins. so i'll just stick with them.