Here's a little road trick...if your mushroom isn't too bad (just feel it and maybe see it, but it's just starting) wet the edge of the tip (really wet it, with saliva), place the shaft (without the butt) flat on the table, and roll it back and forth with heavy pressure on the ferrule with your hand. Keep doing it until you can't feel the mushroom, then burnish the he@$ out of it with a dollar bill. Usually only have to do it once, soon after a new tip is installed...then you shouldn't have any more mushroom issues.
(p.s.: Don't let the room owner see you do this on one of his tables...doesn't hurt the surface/cloth, but it makes 'em nervous to see you do it).