On some reels, mainly those with pillars between the 2 halves of the spool , mono backing has been known to wedge creating pressure and distort the spool causing problems. You are probably OK with machined 1 piece spools but I wouldn't chance it, on reels like Stantons, JW Youngs, Aerials and stuff like that .........No way would I use mono backing.
edit: RDR beat me to it by 1 minute