Gaurd screws should always be tight - or you take the chance on cracking the stock thorugh the recoil lug and/or at the tang.
The only action that shows contrary to this, is one with 3 screws - ie: Winchester M70's. Here, the front and rear gaurd screws are tight and the middle one, only snug.

If a rifle with 2 screws showed better accuracy with one loose, this would mean the bedding is wonky and torquing the action when the screws are tightened. That needs to be repaired. Never shoot a rifle with loose gaurd screws unless you are planning to have to repair some cracks.