In all my years of umpiring, I have never refused a request from a player to move...
That being said I can not disappear and depending on the runners on base I have to be on one side of the mound or another to properly work the game....
I have found that a step left or right and properly going to the "hands on knees" position almost always gives the fielders enough of a view to see/field easily.....
Any umpire that refuses a reasonable request to move is wrong.