I would have to say it depends on a match to match basis. For example of you have a rider like Dayle Wood who (no offence) isn't going to beat anyone, it makes sense to give Fredriksen / Clegg the easier rides to score as many as possible. But of you have a rider like Coles who could have the beating of Roynon in heats 2,4,8 then leave Boxall to beat the second strings in the harder races?