Oh my friends, do I have the solution for you. Best investment you'll ever make for perfect bottoms, and you'll throw those pizza cutters and knives out the window. Now I did say investment, so don't say I didn't warn ya.
Get a pack of those thin, plastic chopping mats from Wal-Mart or whatever, they usually come three to a pack. Cut about a six inch square from one. When you're ready to sever that fondant from the cake, lay your square up against the cake, and cup your hand to make it curve around the tier.With your other hand, gently push down until it slices thru the fondant thru to the table. Repeat all the way around. When you've worked all the way around the cake. Take that same plastic and start moving it up and down rapidly, and clean up the little straggly bits. I also use it as a smoother.
My favorite trick of all!
Um, you are now officially my hero. Well, you already were so I guess now that makes you a Superhero.