MIT holds rush in August, about 2 weeks before classes start. It works pretty well. All freshmen arrive on campus a day or two before rush starts, and rush is about all there is to do for those 4-5 days. On the day that rush starts, all the freshmen are gathered together for the class picture and a motivational speaker... upperclassmen quietly wend their way through the crowd, striking up conversations... then at a signal from the IFC president, rush officially begins, and surprise, all those upperclassmen are suddenly wearing letters.
Since there's nothing happening except rush, and since almost all the rushees are first-semester freshmen who haven't already budgeted a thousand and one extracurricular activities into their schedules and feel they don't have time to join a GLO, rush numbers tend to be pretty good... deferred rush would definitely hurt the system.