The only potential problems you'll run into are that mono, if left on the spool too long, develops a wicked curl due to memory and in a slack line situation (which is something to be avoided anyway) may cause the line to bunch up...also, old mono backing may develop nicks in it over time--and of course a blood knot can get old on your thumb...

Try filling half of your spool or so with braided dacron backing, the stuff flyfishers use. It's got no memory, lays nicely on the reel, and pays for itself after you swap mono tips a couple of times...plus the knots lay rather flat...er...which only really matters if you're using a baitcaster...just lay enough mono on there so you don't feel the knot when your working line is out. I got tired of replacing whole spools of line but carry an extra spool with me--good for 3 or four respools.

I'm unsure about dacron on a spinning reel, though...