In my experience when you're busking people rarely listen for more than a few minutes so I can't see how it would damage uptake for the gig later on. (Especially as people presumably did get a taste of what was coming- albeit in tiny segments.)

Mind you this all presupposes the high traffic street corner aproach rather than the gather in the crowd and bottle square kind of approach- (which is the only one really available to some acrobat/juggler type performers as the impressive part will be quite short rather than a continuous stream of music.)