A rail would certainly allow you more flexibility in positioning your scope for some additional eye relief. The only time you would need a 10, 20 or 30 mOA rail would be if you were going to be dialing long distance and you believe your current or proposed scope will not have enough travel.
Each scope is different as to the amount a elevation travel it will afford you. For example, a Leupold VX-3 4.5-14x40mm CDS with 1" tube has an advertised 71 MOA of maximum travel. Assuming that you were able to zero your rifle at 200 yds and still be somewhere in the middle/center of your scope elevation adjustments (rarely the case!), you would have half, or 35 MOA, of your total elevation travel left to dial up. Now the VX-3 6.5-20X50mm LRT has a 30 mm tube and an advertised 94 MOA total travel. This adds another 23 MOA overall and roughly half, or 12 MOA, in additional elevation adjustments. The Viper Vortex 6.5-20x50mm PA has an advertised total elevation adjustment of 68 MOA. So total elevation travel depends on the make of scope, size of tube and magnification. By putting a scope on a 20 MOA rail, you pick up an additional 20 MOA of total travel in elevation - all of which is usable in dialing up.
So, what does all this mean? Depends on what you are shooting. In my flat shooting 6.5-06 AI, I need 22.25 MOA in order to reach out to 1000 yds. Slower or less aerodynamic bullets will require more. To get out to 1500 yds, I need approx. 49 MOA. of dial up.
So bottom line any decent scope with a standard rail will probably have enough elevation travel to allow you to dial up to 800 - 1000 yds, depending on your caliber. If you want to make sure that you can reach 1000 yds, then get a good scope with at least 60 MOA total travel in elevation. If you want to go beyond 1000 yds, then a MOA rail is probably in order. As a very general rule of thumb, each addl. 10 MOA beyond 1000 yds will buy you another 150-200 yds. in dial up beyond 1000 yds.
Today, nearly all scopes provide you with turets or knobs that have dial up capability. The problem is that a lot of the less expensive scopes do not track well - meaning that they will not hold their zero. This is referred to repeatability. For example, let's say you are zeroed at 200. You dial up 23.5 MOA for a 1000 yd shot. After you make the shot, you dial back down exactly 23.5 MOA. You should have returned to your zero, right? Unfortunately, with a lot of scopes, you will be higher or lower than your zero, because they do not track well - but that is a story for another day.
Good luck in your picatinny rail search.