Using lower than your maximum possible power level gear doesn’t hurt for things like crucible (no level/power advantage) or the power of your engram rewards and drops, but it is technically dragging down your effectiveness in PvE. I don’t know the math but I think it’s sort of like the delta between your overall power level and an enemy is a factor applied to both damage dealt and received against them, and at the high end (raid, prestige nightfalls), it matters more.
I certainly don’t run around with my maximum power loadout all the time—right now that would mean a hand cannon that I don’t really love—but it’s a good idea to keep your favorite gear somewhere close to your maximum. You should at least be infusing it with your current level drops for auto rifles you don’t like every now and then as that power level rises.
Do you know your Base Power Level right now? You can find it with Destiny Item Manager if you don’t know.