I'm certainly no expert, but I'll tell you my experience. I run QRPp whenever it's possible given summer QRN and poor propagation. I don't like the "listen and pounce" approach one bit, so I always call CQ. I will run as low as 100 mW which is as low as my radio can be dialed down to. If I can't get a reply within one or two CQ calls at 100 mW, I start dialing the power up until I do get a response. Depending on conditions, that can go as high as 5W which of course isn't QRPp but then reality is that QRPp isn't always practical in all conditions.
I use a 600 foot delta loop, that is up 35 feet at the feedpoint, and up 50 feet elsewhere. It is fed with balanced feedline (400 ohm from MFJ) which of course virtually eliminates all feedline losses. I'm sure a much smaller loop would work pretty well, too, but obviously not as well as 600 feet does. This antenna is always used with an automatic antenna tuner.
Summer QRN makes QRPp more difficult on the one or two bands that are reliably open at this point in the sunspot cycle, 40 and 30. Last winter, 100 mW worked very well most of the time, even for rag chewing. Now granted, I wasn't making contacts with NZ, but instead contacts in the eastern US and Canada, from SW Ohio. I think my furthest contact was Puerto Rico for QRPp. If I can't get at least a 57 report, I dial up the power a little, as I'm not trying to torture the person on the other end.
Of course, I am working CW since it is really the only thing that works well with QRPp, at least in these conditions.
Just my 2 cents worth, for what it's worth.
Ciao and 73,