Using an SPL meter may not make a difference to your ears, but it will help to make sure that the speakers’ output volume is balanced at the calibrated listening position. The channel level settings, that you adjust in the receiver, may vary, which can be affected by many different variables.
Using the internal test tones of the receiver, and the channel levels initially set to 0dB, you turn the master volume up to a comfortable level (something in the neighborhood of 75db on the SPL meter, which will be your calibration level).
When you start you will need to use one of the speakers as the initial reference point. You then adjust each channel level so that you measure a 75db reading on the meter. The subwoofer is slightly different because its output level can be adjusted by its gain control and/or the sub channel level in the receiver.
You can try this by ear but it won’t be as accurate. Also, if you have a rather current receiver, you could try its auto-setup feature if it has one. Unfortunately, auto-setup usually doesn’t set things up properly.