I would fill every "P" spot possible with a STARTING pitcher. Given the scoring system you mentioned, it's hard to imagine a closer getting more points over the course of a season than a reasonable starter, even a WW or FA starter.
And this is NOT ROTO. Just because Holds is a category doesn't mean you should try to compete in that category. If a "holds" guy produces 100 points over the season and an SP produces 101 points, you want the SP 100% of the time. Your holds pitcher will not outproduce guys who are likely still availible at SP. Even guys like Hudson, Sowers, Igawa, Bonser, Maine, etc., who are all probably on your FA list right now, will probably earn twice as many points on their W-L record alone than any holds guy will earn in the "holds" category.