Unless you include psychological factors, this is the wrong strategy.The score most definitely affects my shot selection and I think it should.
Quite a lot of times you will be faced with chosing between playing an offensive shot and a defensive one, if I am well ahead I tend to go more offensive. If my oppenent is on the hill I will almost always chose a good safety over a less then 90% shot.
gr. Dave
As mentioned by SJM above, at nine ball each rack is a race to one and you should do your very best to win each rack. Your ability to win the next rack is not affected by what happens in the current rack unless you consider the small effect of the break (if you are playing winner breaks).
If you pass up an 89% shot, you better be a 90% favorite to have the next open shot after the safety exchange. If you are only 80% to win the safety battle, you wasted an opportunity.