When it comes to properly controlling airspeed and altitude there are a couple of schools of thought depending on which group of pilots you happen to be talking with. One subscribes to the idea that power management is all that's really needed to control airspeed and altitude assuming the airplane is properly trimmed. The other group holds that airspeed is best controlled with pitch and altitude with power.
Either approach will work, but they have drawbacks as well, especially in turbulence when the slow response of power inputs can play havoc on your attempts to nail a target altitude and airspeed.
A better method is to use a touch of pitch and power in harmony when altitude or vertical speed needs correcting and airspeed is on target. This is known as elevator-throttle coordination, and it's a must for proficient instrument flying by hand.
Here's how you should be thinking about elevator-throttle coordination: If you change the power setting, you must simultaneously adjust pitch to maintain target airspeed. Conversely, if you change pitch, you must simultaneously adjust power to maintain airspeed.
Obviously finesse is the goal when you make these coordinated control inputs since a small pitch change combined with a small power change will generate a significant performance change. Fine adjustments of both will make life much easier for you.