I agree with what the others have said about your maneuver dictating your throttle management - to achieve constant speed and preserve maneuver shape. But I've found that with a lot of planes, if you cut throttle down to idle right at the top of the loop segment, the vertical-down segment tends to be too steep. For that reason, most pilots hold in a bit of throttle until they're about 30-40 degrees nose down, before cutting throttle. This helps to round out the loop. As for gaining speed on the downside, (since you're keeping throttle on a bit longer), that's one reason why pattern pilots like the large props and 4 strokes and also the large body fuselages...for their braking ability on the downline, to preserve that oh so valuable constant speed...

Hope this sheds a bit of light..

Cheers,
Ken