No, Manganin is not used for power resistors, its TCR becomes unstable above about 60°C. Several alloys are used for power resistors, Cupron is the main one with several other alloys such as #60,#90 and #180, all having a fairly high operating temperature as power resistors can have hot spots of up to 350°C, their overall TCRs are usually over 100 PPM/°C. Relative new comers, so-called precision powers, mostly under 5 watts rating and 1% tolerance, have lower TCRs because they are made with Evanohm and have limited operating temperatures, these are under 100 PPM/°C TCRs. The standard power resistor made with lugs strapped around the ends of the ceramic core have inherent high TCRs.
Precision wire wound resistors are an entirely different animal and have little in common with power resistors except that both have wire wound onto a bobbin with leads. Precision can have the lowest TCRs of most precision resistors and have the lowest inherent thermal noise if made correctly. Precisions can be made with power ratings of 2 watts or higher but are not considered power resistors.