Your valves close by spring tension automatically. So as long as the tappets are not holding them open the valves would only effect the compression test if they are not seating completely. If you can fit a feeler gauge when the piston is at TDC compression stroke, then the valve lash is not causing it. You can also just loosen then way out and test compression. This would take the lash out of the variable.
There are seveal variables that could cause a loss of compression. Like a build up of carbon or a holed piston.