Re: 20 moa base necessary or no?
Necessary? It depends, on your rifle, your load, and the scope you choose.
For example:
308 Winchester - most loads need 10-11 Mils (34 to 38 MOA) to reach 1K yds. Mount a scope that has only 13 mils of elevation (45 MOA)like a Bushnell 4200 6-24x50MM Tactical on a 0 MOA base.
Now you have 6.5 Mils up (22 MOA) and 6.5 Mils down (22 MOA). No reaching 1K yds without a hold over, from a 100 yd zero.
Add a 20 MOA (5.5 Mils) base to the above combo, you now have available in theory 12 Mils of elevation (41 MOA), and reaching 1k yds is usually not a problem.
Choose a different scope, like a Fixed power Super Sniper, or a 3.5-15x50mm Night Force for example, you have over 110 MOA on tap, so a 20 MOA base is not really needed, but never hurts.
In short, your rifle, your load, and your scope, plus what you want to accomplish, should drive your choice of bases.
One size does not fit all.
Bob