No you don't need to use +P data in +P cases. Just work up loads for the brass you have.
As with any reloading, if the cases vary much in internal volume I separate cases and work up the loads separately for each batch. I do it for consistency, round to round.
I necked up ~100 or so 6 mm Remington cases I reserve for 75 grain Sierras. My hunting loads with 100 grain NPT and 90 grain Sierra HPBT go in Nosler +P brass. I don't use +P data, just the regular 257 Roberts data at Hodgdon's website. I'm loading them for a small ring Mauser though and 46k C.U.P works just fine. The 6 mm Remington cases weigh less, but not much.
