When mixing 5.1 and 7.1, I switch the instrumental channel from 100% L/R to 70% L/R to balance the soft instrumentals and effects with the center channel. However, this narrows the width a bit. Is this normal and common, or do you have another technique?

The reason I do this is to prevent the center from remaining completely dry in the surround speaker system. I do this so that it doesn't get heard in very different worlds and sounds more harmonious.

I hope it's perfectly normal.

Note: This is for music/song surround.

Instrumental Stereo track send (5.1):


Instrumental Stereo track send (7.1):

If you turndown the center (like you have no LFE) it will be stereo spread over the other speakers.
But what is your goal with 5.1 / 7.1?
They are not stereo.

I know they aren't stereo but i try to say, i want to make sound more wider on Surround.

By separating the audio sources across the available surround channels, it will be as wide as possible.

You probably mean a little delay for widening.