If you get it and never use it, atleast you already have it. If you dont and find out you need it you will spend the money twice. Get the 20 moa as a start.
I've only run into one scope that gave me a problem - a sightron sii 36x fixed - a benchrest type scope. I couldn't quite get it zeroed at 100 yards. For the vast majority of scopes out there, you want at least 20MOA.
Scopes seem to work best in the middle of their adjustment range. When shooting distance, you can quickly be on one end of the tube without a canted base.
I only run 20 MOA Bases being I take the rifles out to some distances beside 2-3-400 yards. Another reason I really like the idea of the 20 MOA is if you scope does not have Zero stops (I own a few 5-20HD SWFA scopes) with a 100 yard zero I only have 10 mils till the scopes bottom out. So I know that from the bottom out only 10 mils and I'm back at zero. But this also depends on the rifle if its flatter shooting or not a flat shooting rifle will also change this number but will keep you in range. Either way a cool way for a known zero for me with the 20 MOA bases.
Also like stated better have it then need it and it does not hurt to have a 20 MOA base.