The most common question about shrub trimming is: "When?" With flowering shrubs, the easiest "rule of thumb" to remember is to always trim flowering shrubs within one month after they finish blooming. That way you won't risk removing next year's blossoms which have already begun to form.
Most shrubs are trimmed following their annual flush of growth. But some shrubs can suffer from late-summer trimming, which can cause a late flush of growth that doesn't properly harden-off before the arrival of winter weather.
The most severe cutting-back of shrubs is best done in early spring, just prior to the emergence of new growth. We've had success rejuvenating large Yews, Junipers, Lilacs, Rhododendrons, and many other varieties during this early-season timeframe.
"Candle" describes well the new growth on Mughos and other Pines (photos above and below). 1/2 to 2/3 of each new candle should be trimmed off after the candle fully elongates, but before the needles elongate - this is usually during June in SW Pennsylvania. Using these methods will keep your pines compact and well shaped.