In talking to a couple of drillers in my area, they all say that for the past four or five years, the trend has been for drillers to suggest relaxed spans. They all agree that most professional bowlers have switched to relaxed spans - it allows bowlers to reduce risk of injuries, less strain on tendons, etc.
I personally went from relaxed to a longer span - I find that I get better release, less hang ups, and better ball reaction with my new stretched span.