This may seem like a strange question.
I hooked up my washing machine to the water supply lines during installation. I did not open the water line 100% I only opened it a bit over half way. 60% open form the wall valves.
I thought this may reduce the fast volume of water slamming into the washing machine. The washing machine seems to turn on and off really fast with its electric servo. My Brain said, if there was not such a high volume of water going into the washing machine it may reduce any water hammering and overall be gentler on the machine?
Reducing the flow of water into the washing machine to help reduce the water from slamming into itself. These washing machines turn on and off so fast.
Not knowing much about plumbing, but trying to use common sense, I thought I would check to see if I am actually creating a problem while trying to solve one?
Does it matter if the supply line is on 100% or 60%?
This is not the main water line, just on the stubbies water line valves for the washing machine.

 

Purchase a pressure gage (0-100 psi) that can be screwed onto a hose connection, Measure the cold water pressure. Check the washing machine's manual for recommended operating water pressure . If the measured pressure is near the recommended, the washing machine supply valves can be fully open. To stop the water hammer, you will have to install water hammer arrest devices on the washing machine supply lines (hot and cold).

 

I argee with beelzebob's post. Adding one more point, your washer may not be operating properly if water pressure is insufficient and washer is not designed for it. Best to check the pressure.

If you have reason to believe water pressure is too high, the issue may be a whole house issue. If so, the fix would best be located at the entry.

 

This will solve your issue with water hammer caused by your washing machine.
https://www.homedepot.com/p/Oatey-Qu...8600/100069256

 

That depends on the actual water pressure but appliance valves require water pressure to close properly and to that end most appliance manuals tell you to open the valves fully.

Measuring the cold water pressure is an excellent idea to see what the house pressure is.
If it's too high..... closing the valve partially will be of no help.
You may need to consider a pressure reducing valve on the water main.

 

I would open them fully, but it also sounds as if you have excessive water pressure. I wouldn't want the pressure to be over 60 psi.