Eric,
I have always used right rudder on the loop entry due to P-factor.
Even some left on the backside.
Are you saying that is incorrect?
Mark, I'd not call that incorrect if your particular airplane (and how you fly it) requires that, and gives you proper results. At typical loop entry speeds in the various airplanes I've flown, I've never noticed a need for right rudder until the over-the-top section where you'll notice some slipstream yaw due to the low airspeed.
When you pitch into a loop, yes there is slight P-factor, but there is also a little gyroscopic precession (in right turning engines) that applies a right yawing force. This is stronger with metal props, of course. This tends to cancel any need for right rudder during the pull into a loop. Some high power-to-weight airplanes running metal props may actually need a touch of left rudder pressure when pitching hard into a loop. And in high-powered airplanes with light composite props, I've never noticed a need for right rudder, but airplanes vary, and your experience may be different. For most pilots, the need for right rudder during loop entry means they are correcting a dragged wing condition due to inadvertently applying a touch of left aileron during the pull. This is very common.
But you also mention left rudder on the backside. I assume this is because you're pulling power off on the backside? At high speed, and low power settings, many airplanes require some left rudder to trim the airplane regardless of any maneuvering. But if you maintain a high power setting throughout the loop, you likely will not need that, but again that is my experience, and your airplane and experience may be different.
I notice lots of folks pulling large amounts of power off (even to idle) on the backside of loops for fear of overspeeding and blowing out low. This is not necessary, and will not lead to nice looking loops. As long as you exit the loop at just about the same altitude as you entered, it's physically impossible to gain airspeed during the loop if your power setting is constant. Don't pull power, just manage your G so that you end up about the same altitude as you entered. Of course, be ready to pull power if you mess it up a little.